Chapter 18 Big Idea : Most everything is a mixture

How Chemicals Mix

Chapter 18

Big Idea : Most everything is a mixture

Most materials we encounter are mixtures: mixtures

of elements, mixtures of compounds or mixtures of elements and compounds.

Examples: stainless steel- mixture of iron (Fe), chromium

(Cr), nickel (Ni) and carbon (C). our atmosphere (air)- mixture of nitrogen (N2),

oxygen (O2), argon (Ar), carbon dioxide (CO2) and water vapor (H2O)

18.1 Most materials are mixtures Mixture- a combination of two or more substances in

which each substance retains its own properties.

There is a difference between the

way materials combine to form mixtures and the way elements combine to form compounds

Mixtures vs. compounds

Each substance in a mixture retains

its chemical identity. Ex- if you mix sugar into a cup of tea,

the tea keeps its properties and the sugar keeps its properties. They merely mix together. The sugar molecules just spread out uniformly throughout the tea.

So, the formation of a mixture is a physical change, not a chemical change.

Mixture

In contrast when you form a compound there

is a chemical change that occurs. Recall that when sodium (a soft, highly

reactive metal) combines with chlorine gas (a green, poisonous gas) to form sodium chloride ( a white, salt that we put on french fries), the elements lose their chemical identities and form something completely different.

Compound formation is a chemical change, not a physical change.

Compound

The components of mixtures can be separated from one another by taking advantage of differences in the components’ physical properties.

ex- you can filter solids out of a mixture of solids and liquids. (this is how coffee is made)

This process is called filtration – takes advantage of size differences between components.

Mixtures can be separated by physical means

ex- you can take advantage of a difference in boiling and melting points.

Saltwater (ocean water/sea water has more stuff in it) is a mixture of water and salt.

To separate the salt from the water, you can boil off the water leaving only salt crystals behind. This process of boiling off liquids at their boiling points is called distillation.

It is a very effective method in separating liquids from each other.

18.2 The Chemist’s Classification of Matter

Pure Substance- material that consists of only one type of element or one type of compound

Mixture – contains two or more elements or compounds mixed together.

Mixtures can be heterogeneous or homogeneous.

Heterogeneous mixture- the different components can be seen as individual substances- ie, pulp in orange juice, sand in water, oil in vinegar, granite

Homogeneous mixture- have the same composition throughout. Uniformly mixed so that you cannot see the individual components. 14 kt gold, sugar in tea, cough syrup.

Suspension- homogeneous mixture in which the

different components are in different phases – ie solids in liquids, or liquids in gases.

The mixing is so thorough that the different components are not easily distinguished

Ex- milk, blood, clouds. One way to tell is by shining a beam of light through

them- the path of the light will be visible One way to distinguish a suspension from a solution is

to run it through a centrifuge which will separate the parts of a suspension but not the parts of a solution.

Suspensions

When you dissolve a solid in a liquid you are

creating a solution. The solid separates from each other and spreads

out evenly throughout the liquid so it appears to disappear. It is only spread out, however.

Terms: Solvent- the component present in the largest

amount (what does the dissolving) Solute(s)- other components present in smaller

amounts (what is being dissolved)

18.3 Making solutions

To make a solution a solute must dissolve in a

solvent

Whether something dissolves in another is a function of their electrical attraction for each other. The stronger the attraction, the greater the ability to dissolve.

There is a limit to how much of a given solute can be dissolved in a given solvent. At that point it is said to be saturated with solute.

Types of solutions

Saturated solution- a solution containing the

maximum amount of solute that will dissolve in its solvent.

Unsaturated solution- a solution that is capable of dissolving additional solute.

The quantity of solute dissolved in a solution is

described in mathematical terms by the solution’s concentration- which is the amount of solute dissolved per amount of solution

Concentration = amount of solute/amount of solvent

18.4 Concentration and the Mole

Solubility depends on the submicroscopic attractions

between the solute particles and the solvent particles The greater the attraction, the greater the solubility. If a solute has any appreciable solubility in a solvent,

it is said to be soluble in that solvent Solubility is also dependent on the attraction of solute

particles to each other. Ex- if you were to dissolve sugar in water, the water

must first pull the sugar molecules away from each other and get in between them. This limits how much sugar can be dissolved in water. Eventually you run out of water molecules.

18.5 Solubility-the ability of solute to dissolve in a solvent

A material that does not dissolve in a solvent to any appreciable extent is said to be insoluble in that solvent. ie- pepper in water

Just because a solute doesn’t dissolve in one solvent doesn’t mean it won’t dissolve in another. ie- sand and glass won’t dissolve in water but will dissolve in hydrofluoric acid (used to etch glass). Styrofoam won’t dissolve in water but will dissolve in acetone (active ingredient in fingernail polish remover)

Solubility changes with temperature

For many solids, their solubility increases with an increase in temperature. This does not hold true for all solids but most will have increased solubility when a solvent is heated.

Ex- you can dissolve more sugar in a glass of hot tea than a glass of cold tea.

Other solids aren’t affected as much (the solubility of sodium chloride- table salt- isn’t really affected by temperature)

The solubility of gases is actually the opposite

of the solubility of solids. The solubility of gases goes down when the

temperature rises and up when the temperature drops. You can dissolve more oxygen in a stream when it’s cold.

Solubility of gases

Making solids smaller increases the surface

area of the solids exposing more to the solvent increasing the rate or speed of solubility.

Stirring solids forces the solute and solvent particles to come in contact with each other more thereby increasing the rate of solubility

Rate of solubility – how quick something dissolves