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Mixing and SeparatingSarah Jones

In everyday life you would only come across a few pure substances e.g. sugar, pure water, pure gold and helium gas. Most products that you use are a mixture of two or more substances e.g. lipstick, soft drink and air.

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A “Gold” one dollar coin is actually made from a mixture of copper, aluminium and nickel.

Chemical and Physical changes

Chemical reaction – new substance is formed. These reactions are not easily reversed e.g. wood burning.

Physical change – no new substances are formed. These reactions can be easily reversed e.g. ice melting.

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Crystallisation

An example of crystallisation is when you separate sugar from water in a soft drink. You heat the soft drink and the water evaporates while the sugar crystallises.

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• Solution – when one substance is dissolved in another (chocolate milk)

• Solvent – substance in which the chemical dissolves (milk)

• Solute – substance that is dissolved in the solvent (chocolate)

• Soluble – substances which dissolve in a liquid are said to be soluble.

• Concentrated – when more solute is dissolved in a solvent the solution becomes more concentrated.

• Saturated – when no more solute is able to be dissolved in the solvent the solution is saturated.

• Substances which do not dissolve in a solvent are insoluble in that solvent.

• Sand is insoluble in water. The sand settles to the bottom of the water forming a sediment.

• When the insoluble substance is dispersed (spread) throughout the liquid, it is called a suspension.

Solubility There is a limit to the amount of solute

that will dissolve in a solution.

When a solution will dissolve no more

solute, it is saturated.

Until it reaches this point, it is

unsaturated.

Most solids are more soluble in warm

water than in cold water. We say that

their solubility increases as the

temperature increases.

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Separating Suspensions

• Decanting: gently pouring off a liquid, leaving

the solid in the container.

• Example - You gently tip the saucepan so that

the water runs out, leaving the peas in the

saucepan.

Centrifugation: separate mixtures by a spinning motion.

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Filtration: a way of separating a solid

from a liquid (or gas) using a filter.

This is similar to separating sand and

gravel using a sieve.

The solution that passes through the filter paper

and collects in the beaker is called the filtrate.

The solid material that remains in the filter

paper is called the residue.

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Separating Solutions Once a solute has dissolved in a solvent

to form a solution, you cannot separate it

by filtration.

The solution simply passes through the filter

paper in the same way that water does.

If a solution consists of a solid dissolved in water, you can separate them by heating.

• The water evaporates — turns into a vapour and seems to disappear into the air — leaving the solid behind. Salt can be obtained from sea water by this method.

If you want to keep the liquid you must trap it

as it evaporates and condense it back to a

liquid. This process is called distillation.

Distillation can also be used to separate two

or more liquids with different boiling points,

e.g. water and alcohol.

Separating Solids

1. If one solid is soluble in water and the other is insoluble, you can add water.

When you filter the mixture, the residue is the insoluble solid. The filtrate contains the soluble solid in solution. It can be recovered by evaporation.

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2. If one solid is attracted to a magnet and the other is not, you can use magnetic separation.

Used in mining.

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3. If one insoluble solid floats on water and the other sinks, you can add water to the mixture and skim off the floating solid.

You can separate sawdust and sand this way.

4. If one solid is heavier than the other, you can use gravity separation.

A good example of this is gold panning. Here the water is swirled about in the pan, allowing the heavy gold to sink and the lighter mud and sand to be washed off the top.

Separating Colours - Chromatography