CHEMISTRY TEXTBOOK

MIXTURES AND PURE SUBSTANCES. SEPARATING MIXTURES

Students book for 8th grades

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CONTENT

MIXTURES.................................................................................................................................................. 2

Heterogeneous mixtures ........................................................................................................................... 2

Homogeneous mixtures ............................................................................................................................ 3

Solutions ............................................................................................................................................... 4

How can you tell if a substance is pure? ...................................................................................................... 5

Separating a solid from a solution ............................................................................................................ 6

Filtration ............................................................................................................................................... 6

Decanting .............................................................................................................................................. 6

Crystallization ...................................................................................................................................... 7

Evaporation .......................................................................................................................................... 7

Solvent extraction ................................................................................................................................. 8

Separating a mixture of two solids ........................................................................................................... 8

By dissolving one of the two solids ...................................................................................................... 8

Separating a solvent from a solution ........................................................................................................ 8

Simple distillation ................................................................................................................................. 8

Separating two liquids .............................................................................................................................. 9

Fractional distillation ............................................................................................................................ 9

Separating funnel ................................................................................................................................ 10

Separating a mixture of colored substances ........................................................................................... 11

Paper chromatography ....................................................................................................................... 11

SUPPLEMENTARY QUESTIONS ........................................................................................................... 12

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MIXTURES A mixture contains more than one substance. The

substances are just mixed together, and not chemically

combined. For example:

Air is a mixture of nitrogen, oxygen, and small

amounts of other gases.

Shampoo is a mixture of several chemicals and

water.

In a mixture, the substances that make up the mixture:

are not in a fixed ratio

Imagine, for example, that you have 250 ml of water and

you add sand to the water. It does not matter whether you

add 20 g, 40 g, 100 g or any other mass of sand to the

water; it will still be called a mixture of sand and water.

keep their physical properties

In the example we used of sand and water, neither of these

substances has changed in any way when they are mixed

together. The sand is still sand and the water is still water.

can be separated by mechanical means

To separate something by “mechanical means”, means that

there is no chemical process involved. In our sand and

water example, it is possible to separate the mixture by

simply pouring the water through a filter.

Something physical is done to the mixture, rather than

something chemical.

We can group mixtures further by dividing them into those

that are heterogeneous and those that are homogeneous.

Heterogeneous mixtures A heterogeneous mixture does not have a definite

composition. Cereal in milk is an example of a

heterogeneous mixture. Soil is another example. Soil has

pebbles, plant matter and sand in it. Although you may add

one substance to the other, they will stay separate in the

mixture. We say that these heterogeneous mixtures are non-

uniform, in other words they are not exactly the same

throughout.

DEFINITIONS

A mixture is a

combination of two

or more

substances, where

these substances

are not bonded (or

joined) to each

other and no

chemical reaction

occurs between the

substances.

A heterogeneous

mixture is one that

consists of two or

more substances. It

is non-uniform and

the different

components of the

mixture can be

seen.

A homogeneous

mixture is one that

is uniform, and

where the different

components of the

mixture cannot be

seen.

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Heterogeneous mixtures can be further subdivided

according to whether it is two liquids mixed a solid and a

liquid or a liquid and a gas or even a gas and a solid. These

mixtures are given special names that you can see in table

3.1.

Phases of matter Name of mixture Example

Liquid-liquid Emulsion Oil in water

Solid- liquid Suspension Muddy water

Gas- liquid Aerosol Fizzy drinks

Gas- solid Smoke Smog

Table 3.1. Types of heterogeneous mixtures

Homogeneous mixtures A homogeneous mixture has a definite composition, and

specific properties. In a homogeneous mixture, the different

parts cannot be seen.

A solution of salt dissolved in water is an example of a

homogeneous mixture. When the salt dissolves, it spreads

evenly through the water so that all parts of the solution are

the same, and you can no longer see the salt as being

separate from the water.

The salt has dissolved in the water, giving a mixture called

a solution. Salt is the solute, and water is the solvent.

The air we breathe is another example of a homogeneous

mixture since it is made up of different gases which are in a

constant ratio, and which can't be visually distinguished

from each other (i.e. you can't see the different

components).

Figure 3.1. Types of mixtures: Homogeneous (sugar

solution) and Heterogeneous (Sand in water)

INTERESTING FACTS

An alloy is a

homogeneous

mixture of two or

more elements, at

least one of which

is a metal, where

the resulting

material has

metallic properties.

For example, steel

is an alloy made up

mainly from iron

with a small

amount of carbon

(to make it harder),

manganese (to

make it strong) and

chromium (to

prevent rusting).

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Solutions

Water is the world’s most common solvent. A solution in

water is called aqueous solution (from aqua, the Latin

word for water).

However, many other solvents are used in industry and

about the house to dissolve substances that are insoluble in

water. For example, acetone dissolves nail polish. Ethanol

dissolves glues, printing inks, the scented substances that

are used in perfumes.

Acetone and ethanol evaporate easily at room temperature

– they are volatile. This means that glues and paints dry

easily.

Pure substances and mixtures

A pure substance has no other substance mixed with it.

In real life, very few substances are 100% pure. For

example, tap water contains small amounts of many

different particles (such as calcium ions and chloride ions).

The particles in it are not usually harmful – and some are

even good for you.

Distilled water is much purer than tap water, but still not

100% pure. It may contain particles of gases dissolved from

the air.

Often it does not matter if a substance is not pure. We wash

in tap water without thinking too much about what is in it.

But sometimes purity is very important. If you are making

a new medicine, or a flavouring food, you must make sure

it contains nothing that could harm people.

An unwanted substance, mixed with the substance you

want, is called an impurity.

Figure 3.2. It is important that each substance present in baby food is pure.

DEFINITIONS

Solution – a mixture

you make by

dissolving a

substance in a

solvent.

Solute – the

substance that you

dissolve.

Solvent – the liquid

in which you

dissolve the

substance

A volatile liquid is

one that

evaporates easily.

This is a sign that

the forces between

its particles are

weak. Volatile

liquids have low

boiling points.

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How can you tell if a substance is pure?

Chemists use complex methods to check purity. But there is

one simple method you can use in the lab: you can check

melting and boiling points.

A pure substance has a definite melting and boiling

points. These are different for each substance. You can

look them in special tables.

When a substance contains an impurity: its melting

points falls and its boiling point rises. It melts and boils

over a range of temperatures, not sharply.

The more impurity there is, the bigger the change in

melting and boiling points. The wider the temperature

range over which melting and boiling occur.

Figure 3.3. Boiling points of pure and impure water.

SEPARATING MIXTURES

In most chemical reactions, several products are made.

These products often need to be separated from unused

reactants or other impurities. We can use several simple

methods to purify a mixture depending on the state of the

substance we want to obtain. (Table 3.2.)

Method of separation Used to separate….

Filtration A solid from a liquid

Crystallization A solute from its solution

Evaporation A solute from its solution

Simple distillation A solvent from a solution

Fractional distillation Liquids from each other

Paper

chromatography

Different substances from a

solution

Table 3.2. Methods of separation.

STUDY TIP

Remember that

pure substances

have definite sharp

melting and boiling

points. Impure

substances melt

and boil over a

range of

temperatures.

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Separating a solid from a solution

Filtration

A solution contains a solid dissolved in a solvent. The

dissolved solid is called the solute. The liquid that

dissolves the solid is the solvent.

An undissolved solid can be separated from a solution or

liquid by passing it through a piece of filter paper in a filter

tunnel. This is called filtration. (Figure 3.4.)

The solution, which passes through the filter paper, is

called the filtrate.

The solid that stays on the filter paper is called the residue.

The solid should be washed with distilled water to remove

any solution between the solid particles. The solid is then

dried in an oven.

Figure 3.4. Filtration

Decanting is simply pouring off the solution. It is suitable

for solids that have very heavy particles – for example, to

separate sand from water.

A centrifuge is a machine which spins test tubes round and

around at very high speeds. The spinning pulls the solid to

the bottom of the tube. You can then decant the liquid from

the solid. To get rid of any solution between the particles of

solid, you can break up the solid, wash the solid with water

and centrifuge again (figure 3.5.).

INTERESTING FACTS

How did gold miners

search for gold?

Beginning in the

late 1840s,

thousands of

prospectors rushed

to California to

search for gold.

One of the

approaches taken

to isolate the gold

from the soil was

called “panning.”

Dirt would be

placed in the pan

and covered with

water. After

thorough mixing,

the pan is gently

swirled to remove

dissolved material

while the heavier

gold settles to the

bottom of the pan.

The gold is then

separated from the

mixture of soil and

water.

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Figure 3.5. Centrifuging.

A centrifuge is used to separate small amounts of solid held in

suspension from the liquid. For example, chalk from water.

Crystallization

Crystallization is used to obtain a crystalline solid from a

saturated solution.

If the solution is saturated, then when the solvent evaporates,

what is left behind cannot hang on to as much of the solute. So

the solute leaves the solution and forms crystals (figure 5.6.).

Figure 3.6. Crystallization.

Evaporation

A solid dissolved in a solution can be separated by evaporating

the solvent.

By heating the solution the solvent (water) evaporates leaving

the solid (salt) behind. This is possible since the two substances

have different boiling points (figure 3.7.).

If the solvent is flammable, like ethanol for example, then

instead of heating directly with a Bunsen burner, the solution is

heated with a water bath to avoid it catching fire.

KEY POINTS

Solids can be

separated from

solutions by

filtration,

decanting or

centrifugation.

Crystals are

formed when a

solution of a

crystalline solid

is partly

evaporated

then allowed to

cool.

Solvent

extraction can

be used to

separate two

solids dissolved

in a liquid.

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Figure 3.7. Evaporation.

Solvent extraction

Solvent extraction can be used to separate two solutes

dissolved in a solvent. This is especially useful if one of the

solutes is volatile (evaporates readily). A second solvent is

used to extract one of the solids from the first solvent. The

second solvent must not mix with the first. For example, we

have a solution of iodine and salt dissolved in water and we

want to separate the iodine. We shake the solution of iodine

and salt with a solvent called hexane. We do this in a

separating funnel. After shaking, the iodine has moved to

the hexane layer. The salt will remain in the water layer.

Separating a mixture of two solids

By dissolving one of the two solids

If you have a mixture of salt and sand, then by placing the

mixture in water, you will find that the salt dissolves but

the sand remains.

If this new mixture is then filtered, the salt in the salty

water solution passes through the filter paper to form the

filtrate and the sand remains as the residue.

All that is now left to do is to heat the salty water, allowing

the water to evaporate leaving behind the salt.

Separating a solvent from a solution

Simple distillation

A solvent can be separated from a solution by evaporating

and then condensing the solvent. This is called distillation

(figure 3.8.).

DEFINITIONS

Filtration is a

method to separate

a mixture of a liquid

and an insoluble

solid by passing it

through paper or

other material,

which only allows

the liquid through.

Filtrate is the liquid

that passes through

a filter.

Decanting is

separating an

insoluble solid from

a liquid by pouring

off the liquid.

Crystallization is the

formation of

crystals as a

solution cools.

Distillation is a

process in which

solvent is separated

from a solute by

heating the

solution. The

solvent boils and

then condenses

and the solvent is

collected.

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The solution is heated in the round-bottomed flask. As it

boils, steam rises into the condenser (this cools the steam

back to water). Eventually the salt (solute) is left behind.

The water collected in the beaker is called distilled water.

Figure 3.8. Simple distillation of salty water

Separating two liquids

Fractional distillation

If two liquids are miscible (they mix together well), they

can be separated using apparatus shown in the figure 3.9.

Figure 3.9. Fractional distillation.

For example, to separate a mixture of ethanol and water.

DEFINITIONS

Fractional

distillation is a

process in which a

mixture of liquids

with different

boiling points is

separated by

distillation.

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Since, ethanol boils at 780C and water at 100

0C, then by

gradually heating the mixture, ethanol and water vapor

rises up the column making the glass beads hot as they

condense on them.

Once the beads are 78oC, the ethanol vapor is forced into

the condenser, whilst the water vapor continues to

condense and drip back into the flask. Meanwhile the

ethanol, now in the condenser, condenses and drips into the

beaker as liquid ethanol.

Fractional distillation in industry

Fractional distillation is very important in industry. It is

used:

In the petroleum industry, to refine crude oil into

petrol and other groups of compounds. The oil is

heated and the vapors rise to different heights, up a

tall steel fractionating column.

In producing ethanol. The ethanol is made by

fermentation, using sugar cane or other plant

material. It is separated from the fermented mixture

by fractional distillation. Ethanol is used as a

solvent.

To separate the gases in air. The air is cooled until it

is liquid, then warmed up. The gases boil off one by

one.

Separating funnel

If two liquids are immiscible, then a separating funnel is

used (figure 3.10.).

Figure 3.10. Separating funnel.

KEY POINTS

Simple distillation is

used to separate

water from a

dissolved salt.

Fractional

distillation is used

to separate more

volatile liquids from

less volatile liquids.

Purification of a

mixture often

involves a

combination of

methods.

INTERESTING FACTS

In Bhutan, steam

distillation is used

to make flavoring

and perfume from

lemon grass.

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For example: If you pour a mixture of oil and water into

the funnel, the oil floats on top of the water. All that is left

to do is for the tap to be opened to allow the water to pour

through. The tap is closed once all the water has passed.

Separating a mixture of colored substances

Paper chromatography

Separating the colored substances in black ink (figure

3.11.)

Figure 3.11. Paper chromatography.

1. A small drop of black ink is placed into the centre of the

filter paper.

2. Water is then dropped onto the ink.

3. The ink slowly spreads out, separating into rings of

different colours.

4. The filter paper with its coloured rings is called a

chromatogram.

5. The coloured substance furthest from the original black

ink spot is the most soluble.

STUDY TIP

When choosing a

method to purify a

mixture, think

about the states

and solubility of the

substances in the

mixture.

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SUPPLEMENTARY QUESTIONS

1. Copy and complete using the words below

centrifugation filtrate mixture residue solid trapped

distillation boiling column lower temperature volatile

An insoluble ________ can be separated from a liquid by filtration or ______. When a

_______ of a solid and a liquid is filtered, the solid _______ on the filter paper is called

the _______ and the liquid, which passes through, is called the ________.

Fractional ________ separates more volatile liquids from less ________ liquids. The

more volatile compounds have _____ boiling points. They move further up the

distillation _________. Each compound condenses when the ______ in the column falls

below its ______ point.

2. Which method or methods can be used to separate the following mixtures?

1) The two volatile liquids ethanol and octanol.

2) A mixture of solid copper (II) sulfate and sand.

3) Water from an aqueous solution of copper (II) sulfate.

4) Crushed chalk from a mixture of chalk and water.

5) Crystals of calcium chloride from a solution of calcium chloride.

3. True/False Questions

Think carefully about the following statements. Are they true or false? Circle your

answer.

a In filtration, the filtrate is always a pure liquid. True/False

b Drinking water can only be obtained from seawater by

distillation.

True/False

c The fractional distillation of miscible liquids is only

possible if the liquids have different boiling points

True/False

d Paper chromatography is a physcial method for

separating mixtures.

True/False

e Mixtures have fixed melting and boiling points. True/False

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4. Mixtures word search

Find 10 words to do with separating techniques.

s o l u t e p v o s c

a f a i h o a i o l h

n m d z a c d l e n r

o i e i u k u r v o o

i x b u n t m a a i m

t t m s i f g t p t a

a u k o b j u e o a t

r r n l l y t u r l o

t e f v s i o d a l g

l d c e o a t i t i r

i l t n a s o s i t a

f i l t r a t e o s p

y c n e u q e r n i h

u c x b k o w g l d y

Use the words you found to complete the sentences below

a) ………………. will help you to separate a mixture of lots of coloured dyes.

b) Air is an example of a …………………..

c) ……………….. occurs when a liquid turns into a gas.

d) You use a filter funnel and paper when you carry out ………………………..

e) A ……………………. is produced when salt dissolves in water.

f) ……………………….. is used to capture the water from ink.

g) The solid left on the filter paper after you have filtered a mixture is called the

……………… and the liquid part of the mixture that drips through the funnel is

called the ……………………

h) Water is sometimes called the universal ………………………….

i) The part of a solution that dissolves is called the …………………..

chromatography

distillation

evaporation

filtrate

filtration

mixture

residue

solute

solvent