5. Is Matter Around Us Pure.doc

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Description:

This set of notes is pertinent to the content given in pages

14 to 15 in the NCERT Science Textbook.

Key words: Atom, element, metal, nonmetal, metalloid, molecule, compound, pure substance, impure

substance, mixture, homogeneous mixture, heterogeneous mixture, solution.

Important key concepts

Atoms are the basic building blocks of matter.

An element is a substance which is composed of single type of atoms.

Elements can be classified as metals, nonmetals and metalloids.

Metals and nonmetals have different properties.

Metalloids have intermediate properties of metals and nonmetals.

Molecule is the smallest particle of a substance which has the properties of that substance and exists in

the free state in nature.

A molecule can belong to an element or to a compound.

A molecule of an element contains two or more similar atoms that are chemically combined together.

A molecule of a compound is formed by combination of different atoms.

A compound is a substance made up of atoms of two or more elements that are chemically combined in

a fixed ratio.

A mixture is a substance which consists of two or more elements or compounds that are not chemically

combined together.

A pure substance is one which is made up of only one kind of particles (atoms or molecules).

Elements and compounds are pure substances.

Impure substances are the ones which contain two or more different kinds of particles.

Mixtures are impure substances.

Mixtures can be classified as homogeneous or heterogeneous.

Homogeneous mixtures are the ones which are completely mixed together and have no visible

boundaries of separation between their components.

Heterogeneous mixtures are the ones in which the constituents remain separate and in which onesubstance can easily be distinguished from the other. It has visible boundaries of separation between

its various constituents.

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A solution is a homogeneous mixture of two or more substances.

Atom

Atoms are the basic building blocks of matter.

An atom can not be broken down into simpler substances by physical or chemical processes.

1. Can you guess?

Which atoms is diamond made up of?

Examples

Water is made up of atoms of hydrogen and oxygen.

Common salt is made up of atoms of sodium and chlorine.

Table sugar is made up of atoms of carbon, hydrogen and oxygen.

Element

An element is a substance which is composed of a single type of atoms.

Robert Boyle was the first scientist to use the term element in 1661.

In the 18th century, Lavoisier defined an element as a basic form of matter that cannot be broken down

into simpler substances by chemical reactions.

There are more than 100 elements known.

92 elements are naturally occurring and the rest are man made. Elements can be classified on the

basis of their physical and chemical properties.

Elements can be classified as metals, nonmetals and metalloids.

Physical properties displayed by metals

Except mercury, all the metals are solid at room temperature. Mercury is liquid at room temperature.

Metals have a shiny surface i.e. they possess lustre.

They have a silvery grey or golden yellow colour.

They conduct heat and electricity.

Silver metal is the best conductor of heat and electricity.

Metals are ductile, i.e. they can be drawn into wires.

For example, copper wires are used in electrical circuits; gold and silver wires are used in embroidery.

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ELEMENTS

NonmetalsMetalloidsMetals


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Metals are malleable, i.e. they can be hammered into thin sheets.

For example, aluminium foil is a very important packaging material. Very thin sheets of silver are used

to decorate sweets.

Metals are sonorous i.e. they make a ringing sound on hitting.

Examples of metals

Sodium, potassium, aluminium, zinc etc.

Physical properties displayed by nonmetals

There are a total of 22 nonmetals, out of which 10 are solid (e.g. carbon, iodine, sulphur), 11 are

gases (e.g. hydrogen, helium, chlorine) and one (e.g. bromine) is a liquid.

Nonmetals have a dull surface.

Usually nonmetals have a dull surface but iodine is an exception it has a shiny surface.

They display a variety of colours.

They are not ductile.

They are not malleable.

They are not sonorous.

Examples of nonmetals: Oxygen, nitrogen, helium, etc.

Metalloids

Metalloids are the elements which have intermediate properties of metals and nonmetals.

For example, boron, silicon, germanium, etc.

Gallium and Cesium are elements that have a low melting point. They turn into liquid form at atemperature that is slightly above the room temperature (303 K).

Atoms do not exist independently. They either combine with the atoms of same element or atoms of different

elements to form molecules.

Molecules

Molecule is the smallest particle of a substance which has the properties of that substance and can

exist in the free state.

A molecule can belong to an element or to a compound.

A molecule of an element consists of two (or more) similar atoms chemically combined together.

For example, molecule of oxygen (O2), molecule of hydrogen (H2)

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O2

O2

O2

O2

O2

Oxygen gas

A molecule of a compound is formed by the combination of two or more different types of atoms that

are chemically combined together.

For example; molecule of sodium chloride (NaCl), molecule of water (H2O)

Compounds

A compound is a substance made up of two or more different types of atoms chemically combined in a

fixed ratio.

For example, water (H2O)

Two hydrogen atoms and one oxygen atom combine to form one molecule of water.

The ratio of different type of elements is always fixed in a compound.

Pure substance

A pure substance is the one which is made up of particles of only one kind. These particles may be

atoms or molecules.

2. Can you guess?

Is air a pure substance?

As oxygen gas consists of only one type of particles, i.e. oxygen molecules. It is a pure substance.

Since sodium metal consists of only one type of particles, i.e. sodium atoms, it is a pure substance.

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Na

Na

Na

Na

Na

Na

Na

Na

Na

Sodium metal

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na

Na


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As water consists of only one type of particles, i.e. water molecules, it is a pure substance.

Elements and compounds are pure substances.

Mixture

A mixture is a substance which consists of two or more elements or compounds that are not chemicallycombined together.

For example, air is a mixture of various gases like oxygen, nitrogen, argon, carbon dioxide, water

vapours, etc. These gases are not chemically combined with one another.

Milk is an impure substance as it is a mixture of water, sugar, fats, proteins, minerals andvitamins.

Sugar solution is a mixture of sugar and water. In this solution, sugar and water are not chemically

combined together.

3. Can you guess?

Is sand a mixture or a compound?

Impure substances

Impure substances are the ones which contain two or more different kinds of particles. These particles

can be atoms or molecules.

We can also say that impure substances consist of two or more pure substances mixed together.

For example, sugar solution consists of two types of particles i.e. water molecules and sugar molecules.

Hence, sugar is an impure substance.

All the mixtures are impure substances because they contain more than one type of particles.

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H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

C12

H22

O11

H2O

C12

H22

O11

C12

H22

O11

H2O

C12

H22

O11

H2O

H2O

C12

H22

O11

H2O

H2O

C12

H22

O11

H2O

C12

H22

O11

C12

H22

O11

H2O


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Characteristics of mixtures

(i) A mixture can be separated into its constituents by simple physical processes (like filtration,

evaporation, sublimation, etc.)

For example, a mixture of sand in water can be separated through filtration.

(ii) A mixture shows the properties of its constituent substances.

For example, air is a mixture. It shows the properties of all the gases present in it. Oxygen and air, both

support combustion; carbon dioxide and air both turn lime water milky, though very slowly.

(iii) Energy (in the form of heat, light, etc.) is usually neither given out nor absorbed in the preparation of a

mixture.

For example, heat and light are neither given out nor absorbed when sugar solution is prepared by

mixing sugar and water.(iv) The composition of a mixture is variable and it does not have a definite formula.

For example, air at different places contains different amounts of various gases, air in metropolitan

cities contains more pollutants (carbon dioxide, sulphur dioxide) and air in mountainous regions, (where

there is a lot of greenery), is rich in oxygen. Hence, air does not have a definite formula.

(v) Mixtures do not have a fixed melting and boiling points. Melting and boiling points vary according to the

composition of the mixture e.g. liquid air does not have a fixed boiling point.

Characteristics of compounds

(i) A compound cannot be separated into its constituents by simple physical processes.For example, water is a compound. It can only be separated into its constituents i.e. hydrogen and

oxygen by the passing of an electric current.

(ii) The properties of a compound are entirely different from its constituents.

Water is a liquid where as hydrogen and oxygen are gases; water does not burn, whereas hydrogen

does; water does not support combustion, where as oxygen supports combustion.

(iii) Energy is usually given out or absorbed during the preparation of a compound.

For example, energy is exchanged when water is prepared by electrolysis of hydrogen and oxygen.

(iv) The chemical composition of a compound is fixed. A compound has a fixed formula.

For example, the composition of water is fixed. One molecule of water consists of two atoms of

hydrogen and one atom of oxygen. Water has a definite formula i.e. H 2O.

(v) A compound has a fixed melting and boiling point.

For example, water has a fixed boiling point of 100 oC under standard pressure.

Types of Mixtures

Take two beakers A and B.

Put some sand in beaker A and some sugar in beaker B.

Add water to both the beakers and dissolve the contents.

We have a mixture of sand and water in beaker A and that of sugar and water in beaker B.

Observe the characteristics of both the solutions.

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Mixture of sand and water

Sand is not completely mixed in water and the two compounds are

distinguishable.

It does not a have a uniform texture since the heavier particles settle down.

It does not have a uniform composition as the concentration of sand particles

is more at the bottom.

It is a heterogeneous mixture.

Mixture of sugar and water

Sugar is completely mixed in water and the two compounds are

indistinguishable.

It has a uniform texture.

It has a uniform composition.

It is a homogenous mixture.

Homogeneous mixtures

The mixtures in which the components are completely mixed together and are indistinguishable from

one another are called homogeneous mixtures.

A homogenous mixture has a uniform composition throughout its mass. It has no visible boundaries of

separation among its various constituents.

Examples

Air

Mixture of salt in water

Mixture of potassium permanganate in water

Heterogeneous mixtureThe mixtures which do not have uniform composition and properties throughout their bulk are called

heterogeneous mixture.

A heterogeneous mixture has visible boundaries of separation between its various constituents.

For example: (i) Mixture of chalk and water

(ii) Mixture of soil and water

All the homogeneous mixtures are called solutions or true solutions.

A solution is a homogenous mixture of two or more substances.

For example, lemonade is an example of solution. This mixture tastes the same through out.

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A

B


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4. Can you guess?

Is milk a homogenous mixture or heterogeneous mixture?

Exercise

Q.1 Why is copper metal used to prepare electric wires?

Q.2 Can carbon be used to make bells? Why?

Q.3. Name one nonmetal which is a good conductor of electricity.

Q.4 Is air a homogeneous or a heterogeneous mixture?

Q.5 Is vinegar a pure substance or an impure substance?

Q.6 Upon mixing corn flour in water, will you get a pure substance?

Q.7 Give one difference between homogeneous mixture and heterogeneous mixture.

Q.8 Give one example of the molecule of the compound.

Q.9 Fill in the blanks.

(i) ___________ are the basic building blocks of matter.

(ii) Broadly, elements can be classified as ___________ and ___________.

(iii) Elements which show the characteristics of both metals and nonmetals are called

___________.

(iv) A molecule can either be of ___________ or ___________.(v) A substance which consists of only one type of particles is called a ___________.

(vi) All the mixtures are ___________ substances.

(vii) ___________ consists of different type of atoms chemically combined together.

(viii) Air is a ___________ but water is a ___________.

(ix) Mixture of sand and water is a ___________ mixture.

(x) Homogenous mixtures are also called ___________.

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ASSIGNMENT

1. Define the following terms:

(i) Atom (ii) Molecule (iii) Compound (iv) Mixture

2. Why is water a compound and not a mixture?

3. Which among the following are metals and nonmetals? Also, state the property on the basis of which

you made your choice.

Chlorine, aluminium, sulphur, sodium, silver, helium, carbon

4. Distinguish between the following:

(i) Heterogeneous and homogeneous mixtures (ii) Metals and non-metals

5. Which one of the following statements is correct?

(i) Air is a compound containing 21% oxygen. (ii) Blood is a compound.

(iii) Mercury is an element. (iv) Milk is a homogeneous mixture.

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Description:



Key words: Solute, solvent, dispersed phase, dispersion medium, true solution, colloids, suspension,

saturated solution, unsaturated solution.

Key concepts

The component of a solution that is present in lesser quantity is called the solute. Solute particles are

also called dispersed particles ordispersed phase.

The component of a solution that is present in larger amount, which dissolves the other component in it

is called solvent. Solvent is also called dispersion medium.

Concentration of a solution can be expressed in terms of solute or in terms of solvent.

A true solution is a homogeneous mixture whose particle size is lesser than 1 nm, it does not show

Tyndall effect, it is stable and its solute particles cannot be separated by filtration. A colloidal solution is a heterogeneous mixture whose particle size ranges between 1 nm to 100 nm, it

shows Tyndall effect, it is stable and its solute particles cannot be separated through filtration.

A suspension is a heterogeneous mixture whose particle size is more than 100 nm, it shows Tyndall

effect, it is not stable and its solute particles can be separated through filtration.

Homogeneous mixtures are called solutions. Some times the term solution is also used forheterogeneous mixtures (colloids and suspensions) which are present in liquid state.

For example, a mixture of starch in water is a heterogeneous mixture but still we use the term

starch solution for it.

A solution in which no more solute can be dissolved at that temperature is called a saturated solution.

A solution in which more quantity of solute can be dissolved without raising its temperature is called an

unsaturated solution.

When a saturated solution is cooled, the solute particles are separated in the form of crystals.

Solubility is the maximum amount of solute which can be dissolved in the given quantity of solvent at

that temperature and at that atmospheric pressure.

Solutions and colloids can be classified on the basis of physical state of dispersed phase and

dispersion medium.

We know that the mixtures are of two types: Homogeneous and heterogeneous.

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Homogeneous mixtures are also called solutions ortrue solutions.

Heterogeneous mixtures can be further classified into suspensions and colloids. But to understand in

detail about true solutions, colloids and suspension we should be clear about the term solute and

solvent.

Solute and solvent

Prepare a solution by dissolving two teaspoons of sugar in a glass of water.

Sugar is present in the lesser quantity, it is called solute in the solution.

Water is present in the larger quantity in the solution, it is called solvent.

Solute: The component of solution that is present in lesser quantity and is dissolved in the larger component

of solution is called the solute. Solute particles are also called dispersed particles or dispersed phase.

Solvent: The component of a solution that dissolves the other component in it and which is usually present in

larger amount is called the solvent. It is also known as dispersion medium.

Example: 100 m of vinegar contains approximately 96 m of water and 4 m of acetic acid. In vinegar,

water is the component which is present in larger quantity, hence, water is called solvent and acetic acid is the

component which is present in smaller quantity, hence, it is called the solute.

1. Can you guess?

What is the solvent in milk?

Concentration of a solution

Concentration of a solution is the amount of solute present per unit mass/volume of the solution.

Consider three sugar solutions A, B and C of 500 m each. Solution A contains 5g sugar, solution B

contains 10 g sugar, and solution C contains 15 g sugar.

If we compare solution A and B then solution A is dilute and solution B is concentrated, as solution B

contains more amount of solute than solution A. If we compare solution B and C then solution B is dilute

and solution C is concentrated as solution C contains more amount of solute than solution B.

Thus we can say that the terms dilute and concentrated are the relative or comparative terms.

Concentration of a solution can be expressed in terms of:

(a) solute (b) solution

Concentration of a solution (in terms of solution)It is the amount of solute present in a given amount (mass or volume) of solution

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=solutionofAmount

soluteofAmount

Concentration of a solution (in terms of solute)

It is the amount of solute present in the given amount (mass or volume) of solvent concentration =

solventofAmount

soluteofAmount

Mostly, the concentration of the solution is expressed in terms of given mass or volume of a solution.

The amount of solution and solute can be expressed in terms of its mass in some cases while in terms of its

volume in others.

Depending upon the quantity used for measuring the amount of solution and solute the concentration can be

expressed in the following ways:

(i) Mass by mass percentage of a solution (m/m) =solutionofMass

soluteofMass 100

In this method, quantity of solute and solution are expressed in terms of their masses.

If it is given that concentration of a solution is 20% (m/m), it means that mass by masspercentage of a solution is 20%. m/m indicates that amount of solute and solution are expressed

in terms of its mass.

For example, 15 g of sugar is present in 100 g of sugar solution. What is the concentration of the solution?Quantity of solute (sugar) = 15 g

Quantity of solution = 100 g

As the quantity of solution and solute are given in grams (mass), we express the concentration in terms

of mass by mass percentage of a solution.

Mass by mass percentage of the given solution =100

15 100 = 15%

(ii) Mass by volume percentage of a solution (m/v) =solutionofVolume

soluteofMass 100

In this way quantity of solute is expressed in terms of its mass and the quantity of solution is expressed

in terms of its volume.

If the sign m/v i.e. mass by volume is used with the concentration of the solution, it meansquantity of solute is expressed in terms of its mass and quantity of solution is expressed in

terms of its volume.

For example, if 10 g of salt is present in 100 m of solution. What is the concentration of the solution?

Quantity of solute = 10 g

Quantity of solution = 100 m

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Amount of solute is given in g (mass) and amount of solution is given in m (volume), hence we will

express the concentration in terms of mass by volume percentage of a solution.

Mass by volume percentage of the given solution =100

10 100 = 10%

Note: Concentration of the solution can also be expressed as volume by volume (v/v) percentage of a

solution and volume by mass (v/m) percentage of a solution . In volume by volume percentage of a

solution, amount of solute and amount of solution are expressed in terms of its volume. In volume by mass

percentage of a solution, amount of solute is expressed in terms of its volume while amount of solution is

expressed in terms of its mass.

Numericals:

(i) If 5 m of carbon dioxide gas is dissolved in 100 m of water. Calculate the concentration in terms of

volume by volume percentage of the solution.

Solution:

Volume of solute (carbon dioxide) = 5 m

Volume of solvent (water) = 100 m

Volume of solution = Volume of solute + Volume of solvent

= 5 m + 100 m

= 105 m

Volume by volume percentage of solution = solutionofVolume

soluteofVolume

100

=105

5 100 = 4.76%

(ii) If 40 g of common salt is dissolved in 320 m of water. Calculate the concentration in terms of mass

by mass percentage of the solution.

Solution:

We have to calculate mass by mass percentage of the solution. Hence the amount of solvent and

solute has to be expressed in terms of their mass.

Mass of salt is given but mass of water is not given.

We know: Density =Volume

Mass

Density Volume = Mass

We know, density of water = 1g/ m

Hence, mass of water = 1 320 = 320g

Mass of solution = Mass of solute + Mass of solvent

= 40 g + 320 g

= 360 g

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Mass by mass percentage of solution =solutionofMass

soluteofMass 100

=360

40 100 = 11.1%

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(iii) If 15% (v/v) solution of vinegar contains 85 m water. How much acetic acid is present in the solution?

Solution:

Volume by volume concentration of solution = 15%

Let the amount of acetic acid present in the solution be x m .

Volume of solution = Volume of solute + Volume of solvent

= (x + 85) m

Volume by volume concentration of solution =solutionofVolume

soluteofVolume 100

15 = 10085x

x

+

15(x + 85) = 100x15x + 1275 = 100x

1275 = 85x

15 = x

Hence, the volume of acetic acid present in the solution is 15 m .

Saturated and unsaturated solutions

Take a glass of water.

Keep on adding sugar to it and keep on stirring it with a glass rod.

A stage will come when no more sugar can be dissolved.

Such a solution is called saturated solution.

Why does the sugar stop dissolving in water?

Sugar stops dissolving in water because all the spaces between the water molecules get filled up.

Raise the temperature of the solution by heating it. Can you dissolve more amount of sugar into the

solution?

Yes, because on increasing the temperature, spaces between the water molecules increase and more

amount of sugar can be dissolved in water.

By performing this experiment two things are clear:

(1) A stage comes when no more solute can be dissolved into a solution at a given temperature.

(2) By increasing the temperature of the solution, more amount of solute can be dissolved in a

solution.

Saturated solution: A solution in which no more solute can be dissolved at a given temperature and

atmospheric pressure is called a saturated solution.

Unsaturated solution: If the amount of solute contained in a solution is less than its saturation level at a

given temperature, it is called an unsaturated solution.

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A solution in which more quantity of solute can be dissolved without raising its temperature and without

changing atmospheric pressure is called an unsaturated solution.

What happens if you cool saturated solution?

When a saturated solution is cooled, the space between the solvent particles is reduced and extra solute

particles come out of the solution and are separated in the form of crystals.

Solubility: It is the maximum amount of solute which can be dissolved in the given quantity of solvent at

particular conditions of temperature and pressure.

For example, the maximum of 32 grams of potassium nitrate can be dissolved in 100 grams of water at 20 oC

and at 1 atm.

Hence, the solubility of potassium nitrate in 100 grams of water at 20oC and at 1 atm. = 32 g

1 atmosphere (atm.) is the pressure exerted by the weight of the air above it at any point on theearths surface. At sea level, the atmosphere will support a column of mercury about 760 mm

high. This decreases with increasing altitude. The standard value for the atmospheric pressure

at sea level in S.I. units is 101.325 Pa.

Now we know what are solutes and what are solvents and now we can study in detail properties of true

solutions colloids and suspensions.

Properties of a true solution

(i) A solution is a homogeneous mixture.

(ii) The particles of a solution are smaller than 1 nm (109 meter).

So, they cannot be seen by a naked eye.

Nanometer is a very small unit of length and symbol of nanometer is nm.1 nm = 109 meter

(iii) When a beam of light is passed through a solution, the path of light is not visible in the solution because

the particles of the solution are very small. So, they do not scatter a beam of light passing through the

solution. In other words, true solutions do not show Tyndall effect.

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Path of the lightnot visible

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(iv) The solute particles cannot be separated from the solution by the process of filtration because the

solute particles in solution are very small. For example, sugar, salt and lemon juice cannot be

separated from lemonade by filtering it.

(v) Solutions are stable i.e., if solutions are left undisturbed the solute particle will not settle down.

Types of solutions/true solutions

Solution can be made by dissolving: solids in solids; solids in liquids; liquids in liquids; gases in liquid; and

gases in gases.

Thus depending on the physical states the solutions can be classified in the following categories:

(i) Solid in a solid: In such solutions both the dispersed particles and dispersion medium are in solid state

e.g. alloys.

Alloys are homogeneous mixtures of metals with metals or metals with nonmetals.

An alloy cannot be separated in its components by physical methods.Then why is alloy considered a mixture not compound?

Alloys show the properties of its constituents e.g. steel which is an alloy of iron also rusts like

iron.

For example, brass is a mixture of an approximately 30% zinc and 70% copper.

Steel is an alloy of iron and carbon. Amount of carbon in steel can vary from 0.1 1.5%.

(ii) Solid in a liquid: These types of solutions have liquids as dispersion medium and solids as dispersed

particles.

Example: A solution of salt in water.

A tincture of iodine: It is prepared by mixing iodine (solid) in alcohol (liquid).

(iii) Liquid in a liquid: In these types of solutions both dispersed phase and dispersion medium are in the

liquid state.

For example, vinegar: In vinegar, water is the solvent and acetic acid is the solute.

(iv) Gas in a liquid: In this type of solution the dispersed particles are in gaseous state and dispersion

medium is in the liquid state.

For example, in carbonated beverages carbon dioxide gas is the dispersed phase and mixture of water

and sugar syrup is the dispersion medium.

In carbonated beverages besides carbon dioxide gas other solutes such as sugar and regentsare also present.

(v) Gas in a gas: In such a solution both dispersed phase and the dispersion medium are in gaseous

state.Air is a solution of gases like oxygen, argon, carbon dioxide and water vapour in nitrogen gas.

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Suspension

A heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the

bulk of the medium. Size of particles of suspension is big enough to be visible by the naked eye.

Properties of a suspension:

Suspension is a heterogeneous mixture.

The size of the particles of a suspension is greater than 100 nm, hence they can be seen by naked eye.

2. Can you guess?

One nanometer is equal to how many centimetres.

The particles of a suspension scatter a beam of light passing through it and make its path visible.

The solute particles settle down when a suspension is left undisturbed. Thus, we can say that

suspension is unstable.

For example, if we mix chalk powder in water and keep the solution undisturbed for sometime, the chalk

particles will settle down.

The solute particles can be separated from the suspension by the process of filtration.

Colloidal solution

The colloids are the type of heterogeneous solutions in which the size of solute particles is intermediate

between size of particles of solutions and suspensions. The particles of the colloids are uniformly spread

through the solution. Colloids are heterogeneous mixtures but they appear to be homogeneous because the

size of the solute particles is very small as compared to suspensions.

For example, milk, starch solution, ink, blood, soap solution etc.

Can you guess?

Is tooth paste a colloid, suspension or a true solution?

Tyndall effect

The particles of the true solution and colloids are not visible by naked eye. How can we distinguish betweentrue solutions and colloids?

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(i) Foam: Colloids in which gas is dispersed in a liquid medium is called foam. For example, shaving

cream, soap bubbles, beer foam.

(ii) Emulsion: An emulsion is a colloid in which minute droplets of one liquid are dispersed in anotherliquid which is not miscible with it. For example, milk, face cream.

(iii) Sol: Sol is a colloid in which tiny solid particles are dispersed in a liquid medium. For example, ink,

soap solution, starch solution, mud, milk of magnesia.

Colloids in which the dispersing medium is gas are of two types:

(i) Aerosols (in which the dispersed particles are in liquid state): They are the colloids in which liquid is

dispersed in a gas. For example, fog, clouds, mist.

(iii) Aerosols (in which the dispersed particles are in solid state): They are the colloids in which solid

particles are dispersed in a gas. For example, smoke, automobile exhaust.

When a gas is dispersed in gas, the resultant will be a solution of gases.

Exercise 1

Q.1 Name two colloids in which the dispersing medium is water.

Q.2 You have been given two solutions, one of which is a suspension, the other is colloid. How will you get

to know which solution is a colloid and which solution is a suspension?

Q.3 Which of the following solutions will scatter a beam of light passing through it: salt solution or soap

solution?

Q.4 What do you mean by 15% (m/m) aqueous solution of sodium hydroxide?

Q.5 Fill in the blanks.(i) ___________ mixtures are also called solution.

(ii) The component of a solution which is present in larger quantity is called ___________.

(iii) Alloys are ___________ mixtures.

(iv) Which of the following is true for colloid, suspension and true solution?

(a) The particles of _________ solution have the smallest size.

(b) Particles of _________ solution can be separated by filtration.

(c) __________ solution can scatter a beam of light passing through it.

(v) Impure substances can be classified as ___________ and ___________.

Now after studying what are pure substances, impure substances, elements, compounds, mixtures, true

solutions, colloids, suspension, we can classify matter on the basis of its chemical properties.

Classification of matter on the basis of chemical properties

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MATTER

Element

Impuresubstance/mixture

Pure substance

Compound

Homogeneous(True solution)

Heterogeneous

SuspensionColloid


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22/46

ASSIGNMENT

1. With the help of an example explain the terms: Solute, solvent and solution. How would you prepare a

saturated solution of sodium chloride in water at 25C? What will happen if this solution is cooled to

10C?

2. Classify following substances into true solutions and colloidal solutions:

Brine, ink, common salt dissolved in water, starch dissolved in water

3. Pick out a suspension from the following:

Sugar solution, milk, blood, muddy water, toothpaste

4. Differentiate colloids and suspensions from true solutions.5. A solute is having 10 gm mass in 100 m of water. Calculate the concentration in (m/m) % for the

solution.

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Description:



Key words: Evaporation, volatile, centrifugation, sublimation, chromatography, distillation, fractional

distillation, crystallization, physical change, chemical change.

KEY CONCEPTS

Heterogeneous mixtures can be more easily separated into its constituents as compared to

homogeneous mixtures.

Different components of mixtures can be separated due to differences in their physical properties.

Evaporation technique is used to separate a volatile component (solvent) from its nonvolatile solute.

Centrifugation is the process of separating the suspended particles of a substance from a liquid by

rotating the liquid at a very high speed.

A mixture of two immiscible liquids can be separated by using a separating funnel.

If a mixture consists of a sublimable and a nonsublimable component, the components can be

separated through sublimation.

Chromatography is the technique of separating two or more solids dissolved in the same solvent in very

small quantities.

The liquids which can mix together in all proportions to form a homogeneous mixture are called miscible

liquids.

If a mixture consists of two miscible liquids that have sufficient difference in their boiling points then the

components of such mixtures are separated by using the technique of distillation.

To separate a mixture of two or more miscible liquids for which the difference in boiling point is less

than 25 K (Kelvin), fractional distillation processes is used.

The different gases of the air are obtained by fractional distillation.

Crystallization is a method used to purify solids. In crystallization process, the pure solid is separated in

the form of its crystals from its solution.

Physical change is the one in which no new substance is formed.

Chemical change is the one in which a new substance(s) is/are formed.

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IS MATTER AROUND US PURE IIIChapter 5


24/46

In the previous lesson we classified matter on the basis of its chemical properties. We have done in previous

lesson that mixtures consist of two or more than two substances mixed together.

Why do we need to separate the constituents of mixture?

We need to separate the constituents of mixture because we need the constituent(s) of the mixture for

our use. For example, we separate cream from milk to prepare ghee.

Secondly, we need to study the nature of constituent(s) of the mixture. For example, colour of the

different food items like blue berries, water melon, turmeric powder, is due to the pigments present in

these foods which help in curing many diseases like cancer, heart attack etc. If food scientists need to

study these pigments, they will have to first separate these pigments from the food items.

Now we will learn the various techniques used to separate the constituents of a mixture.

We know that the mixtures are of two types: Homogeneous and heterogeneous.

To separate the components of a heterogeneous mixture by simple techniques like filtration, handpicking and

sieving can be used.

1. Can you guess?

Why is it easy to separate the constituents of heterogeneous mixture?

It is comparatively difficult to separate the components of a homogeneous mixture. For doing so, special

techniques have to be used.

On what basis are the constituents of mixture separated?

The various constituents of mixture have different physical properties such as density, solubility, size of

particles, behaviour towards magnet, boiling point etc. The difference in these physical properties of

constituents is used to separate them from a mixture.

In this lesson, we will study the various physical processes used to separate the constituents of the mixture.

Separation of dye from ink

Is ink a pure substance?

Take a bottle of ink and keep it undisturbed for several days. After some days, you will see some solute

particles have settled at the bottom.

Hence, we can conclude that ink is not a pure substance. It is made of a solid dye dissolved in a liquid.

How can we separate dye from ink?

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Fill a beaker half with water and place it on a tripod stand and a wire gauge in between.

Put a watch glass on the top of the beaker.

Put a few drops of ink on the watch glass.

Start heating the beaker.

Water from the ink will start changing into vapours.

Continue heating till all the water from the ink changes to vapour state.

The residue left is the dye, from which ink is made.

By evaporation we can separate dye from ink.

Evaporation is used for recovering dissolved solid substance from liquid mixtures (or solutions). Liquid

itself, cannot be recovered by this method. The liquid vaporises and is lost to the air.

What is a watch glass?

Watch glass is a circular, slightly concave piece of glass used in chemistry labs as a surface to

evaporate a liquid or as a cover for a beaker.

It is used as an evaporation surface because it allows closer observation of precipitates.

It is used as a cover for beaker because it does not seal the beaker completely. It prevents

dust or other particles entering the beaker but allows the gas exchange.

Principle behind evaporation

Evaporation is used to separate dissolved solid from a liquid because liquids vaporize easily whereas solids

do not vaporize easily.

Application

Evaporation is used to separate the volatile component from its non volatile solute. For example, the

process of evaporation is used on a large scale to obtain common salt from sea water. Sea water is

trapped in shallow lakes (lagoons) and allowed to stand there. The heat of sun gradually evaporates

water in the shallow lakes and common salt is left behind as a solid.

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How can we separate cream from milk?

You must have observed that when your mother serves you with a glass of milk, if you do not drink that milkimmediately, scum starts floating over the milk. Why?

Scum/cream starts floating over the milk because it is the lightest component of milk.

What are the different varieties of milk available in the tetra packs in the market?

The different varieties are:

Standard milk, full cream milk, toned milk and double toned milk.

Standard milk contains 4.5% fat, full cream milk 6.5 % fat, toned milk 3% fat and double toned

milk 1.5% fat.

How does your mother separate cream from milk?In households, cream is separated from milk by churning the milk.

Why does cream get separated from milk upon churning?

When the milk is churned rapidly the denser particles are forced to the bottom and the lighter particles stay at

the top.

Hence in this way the lighter particles i.e. fats get separated.

This process is known as centrifugation.

But on a large scale, i.e. in dairies, the milk is not churned manually but it is spun with the help of

centrifugation machines.

Centrifugation is the process of separating the suspended particles from a liquid mixture by rotating the

mixture at a high speed.

Applications

It is used in diagnostic laboratories for testing blood and urine samples.

It is used in dairies and home to separate butter from cream.

It is used in washing machines to squeeze out water from wet clothes.

How to separate a mixture of two immiscible liquids?

Do you know about miscible liquids and immiscible liquids?

Miscible liquids are those liquids which can be mixed into each other in any proportion to form a

homogeneous solution.

Examples:

Water and acetic acid are miscible liquids.

Water and alcohol are miscible liquids.

Petrol and kerosene are miscible liquids.

Immiscibleliquids are those liquids which cannot be mixed into each other to form a homogeneous solution.

Examples:

Water and kerosene oil

Water and petrol

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If we have a mixture of kerosene oil and water, let us see how they can be separated.

What is a separating funnel?

It is a laboratory glass ware; it has a shape of a cone surmounted by a hemisphere. It is used to

separate two immiscible liquids on the basis of the difference in their densities.

Pour the mixture of kerosene oil and water in a separating funnel.

Let this mixture stand for some time.

Observation:

A separate layer of oil and water is formed.

Open the stop cork of the separating funnel and pour out the lower layer of water carefully in a beaker.

Close the stop cork of the separating funnel as the oil reaches the stop cork.

At this point, discard some liquid otherwise some traces of heavier liquid will be present in the lighter

liquid when it is collected separately.

Collect the oil in a separate container.

Thus, two immiscible liquids can be separated by using a separating funnel.

When a mixture of mercury, water and benzene is put in a separating funnel, it is separated outin three layers.

Mercury being the heaviest liquid forms the bottom layer; water forms the middle layer and

benzene being the lightest forms the top layer.

Principle

The immiscible liquids separate out due to the difference in their densities.

Applications

It is used for separating the mixture of oil and water.

Iron is extracted from its ore haematite (Fe2O3) by heating coke, calcium carbonate and haematite inblast furnace. During this process, two products are formed, slag (CaSiO 3) and molten iron. Slag and

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Separating funnel

Lighter liquid (kerosene)

Heavier liquid (water)

Stopcork

Beaker

Heavier liquid (like water)


28/46

Cotton plug

Inverted glass funnel

Vapours of ammoniumchloride

China dish wire gauge

Burner

Tripod stand

Mixture of common saltand ammonium chloride

Pure ammoniumchloride

molten iron are immiscible, slag is lighter and molten iron is heavier. Thus, slag floats on top of molten

iron. Slag is removed from the top leaving the molten iron at the bottom.

Separation of a mixture of salt and ammonium chlorideWe have seen in the last chapter that ammonium chloride is sublimable i.e. ammonium chloride changes into

vapours directly on heating and vapours of ammonium chloride changes into solid directly on cooling.

Can you guess?

Give the examples of two solids which aresublimable.

Ammonium chloride is sublimable and salt is nonsublimable. Thus, to separate the mixture of ammonium

chloride and salt we will utilize difference in their physical property.

Experiment:

Take a mixture of salt and ammonium chloride in a china dish.

Cover the china dish with an inverted glass funnel.

Insert a cotton plug at upper end of inverted funnel.

Place the china dish on tripod stand and heat it through wire gauge.

Observations:

White vapours start coming out of the mixture, which are of ammonium chloride. They get collected along the

inner walls of the funnel on coming in contact with them.

Pure ammonium chloride collects on the inner sides of the funnel in the form of a sublimate and common salt

remains behind in the china dish.

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Waterrisesuptakingdyes

alongwith

it

Dye with the highestsolubility

Dye with the lowestsolubility

This process of separating a sublimable volatile component from a non sublimable impurity is known as

sublimation.

Separation of dyes present in inkWe have seen that we can separate the solid component of ink (dye) using evaporation.

Actually the ink is not made up of a single dye, it is made up of mixture of dyes.

Now we will learn to separate the different colours of the ink.

Process:

Take a thin strip of filter paper.

Draw a line on it using a pencil, approximately 3 cm above the lower edge.

Put a small drop of ink from sketch pen or fountain pen (ink should be water soluble) at the center of the

line.

Let the ink dry.

Lower the filter paper into a jar or a beaker containing enough amount of water so that the drop of ink

on the filter paper is just above the water level.

Leave the contents undisturbed.

Observation

Water gradually rises up in the filter paper, taking the dye along with it.

Different coloured spots will be observed at different heights.

Principle

All the dyes present in the ink are water soluble, but the solubility of all the dyes in water is different. The dye

which is more soluble in water rises faster and produces coloured spot on the paper at a higher position. The

less soluble dye dissolves a little later, rises slower and forms coloured spot at a lower height. In this way, all

the dyes present in black ink get separated forming different coloured spots.

This technique of separating two or more solids dissolved in the same solvent is called chromatography.

A special type of paper called chromatography paper is used for carrying out separations bychromatography.

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30/46

This separation is based on the fact that two or more solids are dissolved in the same liquid and the solubility

of the solids in the liquid is different.

The term chromatography is derived from the Greek word kroma meaning colour.

Applications

It is used to separate colour in dyes.

It is used to separate pigments from natural colours. For example, the colour of the carrot is due to the

presence of the pigments: lycopene, beta carotene and alpha carotene. These pigments can be

separated from carrot using chromatography.

It is used to separate drugs from blood.

Separation of two miscible liquids

Those liquids which mix together in all proportions to form a homogeneous mixture are called miscible liquids.For example; alcohol and water, acetic acid and water, petrol and diesel

Mixture of miscible liquids cannot be separated by using separating funnel. They can be separated either by

distillation or fractional distillation.

Distillation

Distillation is a process of heating a liquid to form vapours, and then cooling the vapours to get pure liquid.

The liquid obtained by condensing the vapours is called distillate.

We cannot use distillation for the separation of all miscible liquids. It can only be used if

(a) the mixture consists of only two miscible liquids.

(b) the difference between the boiling points of the liquids is more than 25 K.

(c) the liquids do not decompose on heating.

For example, boiling point of acetone is 56oC and boiling point of water is 100oC. Acetone and water does not

decompose on heating and the difference between the boiling point of acetone and water is more than 25 K.

Hence, a mixture of acetone and water can be separated by distillation.

Process:

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Distillate

LiquidHeating

VapoursCooling Liquid

Acetone andwater mixture

Acetonevapours

Pure acetone

Water out(to sink) Water

condenser

Distillation flask

Wire gauze

Burner

Tripod stand

Water in(from tap)

Thermometer


31/46

Take a mixture of acetone and water in a distillation flask.

Fit the flask with a thermometer.

Heat the mixture slowly.

Observation:

When the temperature reaches 56C, acetone vaporizes. All the acetone vapours condense in the

condenser and can be collected from the condenser outlet and water is left behind.

What is water condenser?

A water condenser is a long glass tube surrounded by a wider glass tube which is having an

inlet and outlet for water.

Principle:

In distillation process, two liquids can be separated if they have a sufficient difference in their boiling

points. The liquid having lower boiling point distils first and the liquid having higher boiling point distils

later.

Fractional distillation

If a mixture consists of more than two miscible liquids and if the difference in the boiling point of the liquids is

less than 25 K then fractional distillation process is used to separate the components of such mixtures.

Fractional distillation is a special type of distillation. In fractional distillation different components of the

mixture are separated due to the difference in their boiling points.

The apparatus used in fractional distillation is similar to that of apparatus used in distillation, except that

a fractionating column is fitted in between the distillation flask and the condenser.

A simple fractionating column is a tube packed with glass beads, the beads provide surface for the

vapours to cool and condense repeatedly.

For example, boiling point of alcohol is 78 oC and the boiling point of water is 100 oC. The difference between

the boiling point of alcohol and water is less than 25 K, hence they cannot be separated by distillation. They

have to be separated by fractional distillation.

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Process:

Mixture of alcohol and water are heated in a distillation flask which is fitted with a fractionating column.

When the mixture is heated, both alcohol and water form vapours as their boiling points are

approached.

The alcohol vapours and water vapours rise up in the fractionating column. The upper part of the

fractionating column is cooler, so as the hot vapours rise up in the column, they get cooled, condense

and trickle back in the distillation flask.

When the vapours condense they release heat. As the vapours condense repeatedly, the temperatureincreases.

After some time, a temperature gradient is created in the fractionating column as the temperature at the

bottom of the column is much higher than the temperature at the top of the column.

When the temperature at the top of the fractionating column reaches 78oC (which is the boiling point of

alcohol), the vapour gets cooled and collects in a beaker kept at the other end of the condenser.

The alcoholwater mixture is kept boiling at such a rate that the thermometer shows the reading 78 oC

which is the boiling point of alcohol. The temperature is maintained at 78oC so that no water distils over.

When the alcohol fraction is collected, the flask is heated more strongly so that the thermometer shows

a temperature of 100oC, which is the boiling point of water.

When the temperature at the top of the fractionating column becomes 100oC, water vapour passes into

the condenser, gets cooled and condenses. This pure water is collected in another beaker as the

second fraction.

This water obtained is known as distilledwater.

Applications of fractional distillation:

1. Crude oil and petroleum is a mixture of kerosene, petrol and diesel. Kerosene, petrol and diesel are

obtained by the fractional distillation of crude oil.2. Air is a mixture of different gases like nitrogen, oxygen, carbon dioxide, argon. All these gases have

different boiling points; the various gases of air are separated from one another by the fractional

distillation of liquid air.

Fractional distillation of air

First the air is filtered to remove dust.

Then the air is compressed to a high pressure and then cooled.

When the air is cooled, carbon dioxide changes into solid and is separated as dry ice. Then this coldcompressed air is allowed to expand. When cold compressed air expands, the temperature decreases

so much that air changes to liquid.

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Air

Compression, cooling, andexpansion

Liquid air

Removal of CO2

as dry ice

Allow to warm up slowly infractional distillation column

Gases get separated at differentheights and temperatures

Boils first

Nitrogen (B.P 196C)Boils

second

Boils third

Oxygen (B.P 183C)

Argon (B.P 186oC)

First to beconverted into gas

Second to beconverted into gas

Third to beconverted into gas

Then the liquid air is fed into the fractional distillation column and warmed up slowly.

Liquid nitrogen has the lowest boiling point of 196oC hence it forms the vapour first and is collected

first.

Liquid argon has a slightly higher boiling point of 186oC. Hence, it is collected second in the

fractionating column.

Liquid oxygen has a still higher boiling point of 183oC .Hence, it is collected last.

Flow diagram for fractional distillation of air

Obtaining pure copper sulphate from an impure sample Take some impure copper sulphate; dissolve it in minimum amount of water in a china dish to make

copper sulphate solution.

Filter the solution to remove insoluble impurities.

Heat the solution gently in a water bath. When a saturated solution of copper sulphate is formed then

stop heating.

When we are heating copper sulphate solution, how do we come to know that the solution hasbecome saturated?

Keep on dipping a glass rod in hot solution from time to time, when small crystals form on the

glass rod, the solution is saturated.

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Coppersulphatecrystals

Allow the hot saturated solution of copper sulphate to cool slowly.

Observation:

When the saturated solution of copper sulphate is cooled, crystals of pure copper sulphate are formed

and impurities remain behind in the solution.

These pure copper sulphate crystals are separated from solution by filtration and then they are dried.

Thus, when a saturated solution is cooled, the solute particles are separated out in the form of crystals.

This process of obtaining pure solid in the form of its crystals from its solution is called crystallization.

Why is crystallization technique better than simple evaporation? Some solids decompose or some gets changed (sugar) on heating to dryness during evaporation. This

problem is solved in crystallization process.

The soluble impurities do not get removed in the process of evaporation but such impurities get

removed in crystallization.

Applications of crystallization

It is used for the purification of salt which we get from sea water.

It is used to obtain pure crystals of alum (phitkari) from impure samples.

Water purification system in water works

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In the cities, the drinking water is supplied from water works. In water works various methods like

sedimentation, decantation, loading, filtration and chlorination are used.

The source of water supply in a city is either a nearby river or a lake. The purification of the river or lake water

is done in the following steps:

1. First the water is pumped out into a large reservoir called sedimentation tank. The water is allowed to

stand in the sedimentation tank for some time. During this time, many of the insoluble substances

present in water settle down at the bottom of the tank.

2. Then the water is sent to the loading tanks. In the loading tank, some alum is added to water. The

heavy particles of dissolved alum deposit on suspended clay particles in water. Thus, clay particles

become heavy and settle at the bottom of the tank. Thus, in the loading tank clay particles are removed

from the water.

3. Then the water is passed through filtration tank. The filtration tank has three layers: fine sand layer at

the top, coarse sand layer in the middle and gravel (tiny stones) as the bottom layer. These layers of

sand and gravel act as filter. When water passes through these layers the small unsuspended particles

which could not be separated in sedimentation tank and loading tank get separated in filtration tank.

4. Then the water is passed into chlorination tank. In the chlorination tank, the water is disinfected, i.e. it is

made free from microorganisms. In the chlorination tank chlorine, is added to water to kill the germs

present in it.

Now this clean water is supplied to homes.

Flow chart for the water purification system

Physical change and chemical change

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Reservoir(lake, river)

Sedimentationtank

Loading tank

Filtration tank

Chlorination tank

Insoluble impuritiesremoved

Clay particles removed

Small suspendedimpurities removed

Germs removed

Clean water


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Physical properties of matter are those properties which can be observed and specified. For example,

colour, hardness, rigidity, fluidity, density, melting point and boiling point are physical properties.

Chemical properties of a substance include composition, structure of a substance and mode of reaction

of that substance with other substances.

Let us see the physical and chemical properties of water.

Physical properties

Water can flow.

Water is colourless.

Water is odourless.

Boiling point of water is 100oC.

Melting point of water is 0oC.

Chemical properties

Water is made up of hydrogen atoms and oxygen atoms.

The water molecules are tetrahedrally arranged.

Water reacts with metals to liberate hydrogen gas.

2Na + 2H2O 2NaOH + H2

Can you guess?

Name one physical change which is permanent.

Physical change

Physical changes are those changes in which no new substance is formed.

For example, when ice melts into water no new substance is formed.

Ice consists of H2O molecules as well as the water.

Most of the physical changes are reversible and temporary. For example, boiling of water is a physical

change. On heating, water changes into vapour and vapours on cooling will change back to water.

During a physical change very little heat (or light) energy is usually absorbed or given out. For example,

when sugar is dissolved in water negligible amount of heat is produced due to friction.

Can you guess?

Name one physical change in which heat energy is absorbed.

Chemical change

A chemical change is the one in which a new substance is formed.

For example:

(i) Burning of magnesium: When magnesium is burnt in air, magnesium reacts with the oxygen of air to

form magnesium oxide. During this change a new substance is formed. Hence this change is a

chemical change.

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(ii) Souring of milk: In summers, when milk is kept at room temperature it turns sour because the

enzymes of microorganisms convert milk sugar to acid.

The milk sugar is called lactose and when the enzymes act on lactose they convert lactose tolactic acid.

In this change, a new substance is formed; hence this change is a chemical change.

Usually the chemical changes are permanent and irreversible.

For example, curd is prepared from milk but we cannot get back milk from curd.

In most of the chemical changes, lot of heat energy is absorbed or given out.

For example, when magnesium burns in air lot of heat energy is given out.

Exercise 1

Q.1 Which method is used to separate alcohol and water from their mixture?

Q.2 What is the difference between the apparatus used for distillation and fractional distillation?

Q.3 How will you separate different dyes present in the ink?

Q.4 What is crystallization?

Q.5 Why chlorine is added to water in the water purification system?

Q.6 Give any one chemical property of water.

Q.7 Give one example of chemical change in every day life.

Q.8 Which gas is obtained first at all during fractional distillation of air?

Q.9 To purify the crystals of copper sulphate, crystallization technique is used. Why cannot the crystals of

copper sulphate be separated by evaporation?

Q.10 How will you separate the mixture of sand and sulphur?

Q.11 Which technique will you use to separate the mixtures of mercury, oil and water?

Q.12 You are given a mixture of sand, water and mustard oil. How will you separate the three?

Q.13 Fill in the blanks.

(i) _________ process is used in dairies to separate cream from milk.(ii) _________ process is used to separate dye from ink.

(iii) Impure crystals of copper sulphate are purified by _________.

(iv) It is easy to separate the constituents of _________ mixture but it is difficult to separate the

constituents of _________ mixture.

(v) _________ is a special type of distillation.

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ASSIGNMENT

1. Name the physical processes commonly used in laboratories, industries to separate mixtures.

2. What type of mixtures can be separated by fractional distillation?

3. Differentiate between physical and chemical changes.

4. Explain the process of separation of air into its constituents.

5. Name the process used to obtain pure solid from its impure sample. Describe the process taking

impure sample of sugar as an example.

WORKSHEET - I

1. The properties of the product after a chemical change are different from those of the constituents.

State whether this product is an element, compound or a mixture.

2. Which one of the following processes gives a compound?

(i) Sodium is burnt in air.

(ii) Chalk and marble chips are grounded together.

(iii) Mixing of sulphur with sodium chloride.

(iv) Addition of alcohol in water.

3. The particles of a mixture are approximately 4050 nm in diameter. Can the constituents of the mixture

be separated by centrifugation or filtration?

4. What is the important criterion to use chromatography for separating components of a mixture?

5. If we put a piece of stone in water contained in a graduated cylinder, water level rises. But on adding

sugar there is no rise in water when dissolved. Explain the two situations.

6. (a) Give reason for each of the following:

(i) Ripening of a fruit is a chemical change.

(ii) Crystallization of a substance from its solution is a physical change.

(iii) Formation of curd from milk is a chemical change.

(iv) Separation of gases from air is a physical change.

(b) Classify the following as a chemical or physical change:

(i) Cutting of trees.

(ii) Melting of butter in a pan.

(iii) An almirah gets rusted.

(iv) Water boils to form steam.

(v) Electric current is passed through water and it is broken down into hydrogen and oxygen

gas.

(vi) Dissolving salt in water.

7. Differentiate between elements, compounds and mixtures. Classify the following into elements,

compounds or mixtures:

Air, Hydrogen peroxide, Sugar solution, Zinc, Germanium, Water, Nitrogen, Sand, Icecream, Washingsoda, Limestone, and Diamond

8. What is a pure substance? How is it different from an impure substance?

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9. Why does not air have a fixed formula whereas water has?

10. Two beakers A and B both half filled with water were taken. 50 g sugar was added to beaker A and 50g

sand was added to beaker B. What is the difference between the two mixtures so obtained?

State the type of mixtures formed in the two beakers.

WORKSHEET - II

1. Beakers A, B and C contain 500 ml sugar solution each. A contains 10 g sugar, B contains 20 g sugar

and C contains 30 g sugar.

Which of the beakers contains the most diluted solution and which of the beakers contains the most

concentrated solution?

2. 50 ml vinegar was added to 250 ml water. Calculate the concentration of the solution in terms of volumeby volume percentage.

3. If 20 g glucose is dissolved in 500 ml distilled water, calculate the concentration in terms of

(a) mass by volume.

(b) mass by mass percentage of the solution.

4. Differentiate between saturated and unsaturated solutions. How can we add some more solute to a

saturated solution prepared at room temperature?

5. What is a true solution? Give its properties. Give an example of a true solution of

(A) solid in solid. (B) solid in liquid. (C) liquid in liquid.

(D) gas in liquid. (E) gas in gas.6. Distinguish suspensions from colloids on the basis of their properties.

Classify the following into true solution, suspension or colloid:

milk, tincture of iodine, carbonated beverages, air, aerosols, smoke.

7. Define tyndal effect. What type of solutions exhibit tyndal effect?

8. Classify the following colloids on the basis of their dispersing medium; (solid/liquid/gas) milk of

magnesia, smoke, gemstone, mud, milk, shaving cream, mist, sponge, foam, fog.

9. What is chromatography? How is chromatography used to separate various dyes present in the ink?

State the principle behind the separation process. Give two more practical applications of

chromatography.

10. Define fractional distillation. How is it useful in separating components of two miscible liquids where

simple distillation fails to do so?

11. Define crystallization. How is it better than simple evaporation? Give two applications of crystallization

in separation procedures.

12. Fill in the blanks

(a) Colloids are _______ mixtures but true solutions are _______ mixture.

(b) Milk is a ________ solution but vinegar is a ________ solution.

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To To prepare

(a) a mixture, and

(b) a compound

uUsing iron filings and sulphur powder, and distinguish between these on the basis of:

(i) appearance,that isi.e., homogeneity and heterogeneity.

(ii) behaviour towards a magnet.

(iii) behaviour towards ccarbon disulphide (a solvent).

(iv) effect of heat.

MCQs I

1. The colour of iron filings is:

(a) blue (b) green (c) greyish black (d) yellow

2. Which of the following is not attracted by magnet?

(a) Iron (b) Iron sulphide

(c) Iron Sulphate (d) Iron filings

3. The smell of H2S is:

(a) pleasant (b) as of rotten eggs (c) as of burning sulphur (d) pungent

4. When iron and sulphur are heated at high temperature,

(a) a yellow coloured iron sulphide is formed. (b) a black coloured iron sulphide is formed.

(c) a mixture of iron and sulphur is obtained. (d) they do not react.

5. When we start heating a mixture of sulphur powder and iron filings, we observe that:

(a) sulphur starts melting (b) iron fillings start melting

(c) mixture becomes red hot (d) mixture evaporates

6. A student, by mistake mixes iron filings and sulphur powder. He wants to separate them from each

other. The method he is advised to use is to dissolve the mixture in

(a) boiling water (b) cold water (c) carbon disulphide (d) kerosene

7. Carbon disulphide is added in a test tube containing a mixture of iron fillings and sulphur. What is the

observation after shaking the test tube well?

(a) Some brown gases are evolved.

(b) Sulphur dissolves to form colourless solution and iron fillings settle down.

(c) A yellow solution is formed and iron filings settle down.

(d) After sometime, carbon disulphide, sulphur and iron fillings form three separate layers in the test

tube.

8. When magnet is rolled in the compound iron sulphide,

1. iron particles are attracted towards the magnet

2. iron sulphide clings to the magnet

3. iron sulphide is repelled

(a) Only 1 (b) Both 1 and 2 (c) Only 3 (d) None of these

_________________________________________________________________________________



41/46

9. The process of evaporation is fastest when the mixture is:

(a) heated but not covered (b) heated but covered

(c) covered but not heated (d) Neither heated nor covered

10. When a mixture of iron filings and sulphur is strongly heated in a china dish, the colour of the product

obtained is:

(a) grey (b) orange (c) black (d) pale yellow

11. When a mixture of iron filings and sulphur is put in carbon disulphide in a test tube, we observe that:

(a) yellow particles of sulphur get dissolved in it

(b) grey particles of iron get dissolved in it

(c) both iron and sulphur particles get dissolved in it

(d) neither iron nor sulphur particles get dissolved in it

12. One of the physical properties of H2S is that

(a) it is a poisonous gas that causes headache and dizziness

(b) it is nonpoisonous

(c) it is pleasant smelling

(d) it is greenish yellow in colour

13. Add carbon disulphide to (i) mixture of iron filings and sulphur (ii) iron sulphide and choose the correct

observation out of the following:

(a) Sulphur dissolves in CS2 in (i) but in iron sulphide does not.

(b) Iron dissolves in CS2 in (i) but iron in sulphide does not.(c) Iron sulphide dissolves in CS2 whereas sulphur does not.

(d) Both sulphur and FeS are soluble in CS2

14. Observe the following diagrams and choose the correct statement:

(a) All of them represent mixtures.

(b) All of them represent compounds.

(c) A and C are mixtures; B and D are compounds.

(d) A and B are mixtures; C and D are compounds.

15. If we pass H2S gas through copper sulphate solution, we observe:

(a) no reaction (b) a black precipitate of CuS

(c) a yellow precipitate of CuS (d) a blue precipitate of CuS

16. If lead acetate paper is brought near H2S gas, it

(a) shows no effect (b) turns black (c) turns yellow (d) turns green

17. While heating iron filings and sulphur, keep your eyes away from vapours because:

_________________________________________________________________________________


A B C D


42/46

(a) sulphur vapours may cause irritation in eyes. (b) sulphur vapours are harmless.

(c) iron vapours may cause irritation in eyes. (d) H2S gas will cause irritation in eyes.

18. Four groups of students record their observations on the gas produced in the experiment represented

by the diagram below. Choose the correct set of observations from the following table. Note that the

positive responses are shown by and negative by signs respectively.

Colour of the

gas

Odour of the

gas

Flammability Action on lead acetate

paper

(a)

(b)

(c)

(d)

19. Take a mixture of iron filings and sulphur powder in a watch glass and iron sulphide in another watch

glass. Now move a bar magnet over both the watch glasses. The correct observation would be:

(a) bar magnet attracts iron filings from one watch glass and iron sulphide from the other watch glass.

(b) only iron sulphide is attracted by the bar magnet.

(c) bar magnet attracts all the substances present in the two watch glasses.

(d) bar magnet attracts only iron filings.

20. What precaution has to be taken with carbon disulphide in a laboratory?

(a) Keep it away from flame. (b) Keep it away from carbon.

(c) Keep it away from distilled water. (d) Keep it away from iron sulphide

_________________________________________________________________________________


Dilute hydrochloric acid

Iron sulphide


43/46

To carry out the following reactions and classify them as physical or chemical changes.

a. Iron with copper sulphate solution in water.

b. Burning of magnesium in air.

c. Zinc with dilute sulphuric acid

d. Heating of copper sulphate

e. Sodium sulphate with barium chloride in the form of their solutions in water.

MCQs II

1. A change is called chemical change, if there is change in

(a) size (b) state (c) shape (d) Chemical composition2. In a chemical change energy is

(a) always produced (b) always used

(c) neither produced nor used (d) produced or used

3. In the given test tube we observe that

(a) There is no reaction

(b) The solution turns colourless from blue

(c) The solution turns greenish blue from blue

(d) The solution turns greenish blue and Cu gets deposited over Iron

4. On burning magnesium in air

(a) Energy is not evolved.

(b) No new substance is produced.

(c) Magnesium combines with CO2 to form MgCO3.

(d) Magnesium combines with oxygen to form magnesium oxide a new powdery substance

5. In the test tube we observe that

(a) a gas in liberated which burns with a pop sound

(b) a gas is produced which turns lime water milky

(c) there is no reaction(d) a gas is produced which extinguishes a burning candle

_________________________________________________________________________________


CuSO2 Sol.

Iron nail

dil. H2

SO4

Zn


44/46

ZnSO4

Fe

6. In the test tube we observe that

(a) CuSO4 crystals melt

(b) water vapours are given out

(c) the colour remains blue

(d) it turns into white amorphous powder

7. On adding BaCl2 (Sol.) to Na2SO4 (Sol.) we observe that

(a) there is no reaction

(b) white precipitates of NaCl is formed(c) white precipitates of both BaSO4 and NaCl are formed

(d) white precipitates of BaSO4 is formed white NaCl is soluble and is in solution form

8. What will happen if we add aluminium in the test tube containing copper sulphide solution?

(a) Colour of solution changes from blue to colourless and reddish brown copper metal gets deposited.

(b) No reaction

(c) The colour becomes pale green and copper metal gets deposited.

(d) The solution becomes colourless but no metal gets deposited.

9. What happens in the test tube shown here?

_________________________________________________________________________________


Heat

CuSO4

BaCl2

Sol.

Na2SO

4Sol.

CuSO4

Al


45/46

Al2(SO

4)

3

Zn

(a) The solution becomes green and iron metal gets deposited.

(b) The solution becomes colourless and iron metal gets deposited.

(c) No reaction takes place because Fe is more reactive than Zn.

(d) No reaction takes place because Fe is less reactive than Zn.

10. The observation in the reaction shown in the below diagram is:

(a) the solution becomes colourless and AI metal gets deposited

(b) the solution becomes green

(c) no reaction takes place because Zn metal is less reactive than Al

(d) no reaction takes place because Al metal is less reactive than Zn

11. Some crystals of copper sulphate were dissolved in water. The colour of the solution obtained would be

(a) green (b) red (c) blue (d) brown

12. A student puts an iron nail in each of the four test tubes containing solutions of zinc sulphate, aluminium

sulphate, copper sulphate and iron sulphate. A reddish brown coating was observed on the surface of

iron nail which was put in the solution of

(a) zinc sulphate (b) iron sulphate (c) copper sulphate (d) aluminium sulphate

13. When an aluminium strip is kept immersed in a freshly prepared ferrous sulphate solution taken in a

test tube, the change which is observed, is that

(a) the green solution slowly turns brown.

(b) the lower end of the test tube becomes slightly warm.

(c) a colourless gas with smell of burning sulphur is observed.

(d) light green solution changes to blue.

14. A piece of granulated Zinc was dropped into Copper sulphate solution. After some time, the colour of

the solution changed from

(a) light green into blue (b) blue into colourless

(c) light green into colourless (d) blue into yellow

15. Solutions of ferrous sulphate, zinc sulphate, copper sulphate and aluminium sulphate were separately

taken in four test tubes and some iron nails were placed in each of the solutions. After

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