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There are different ways to separate mixtures, eg by filtration, crystallisation, distillation or chromatography. The method chosen depends upon the type of mixture.

Play an Atomic Labs experiment exploring how to separate salt and sand.

You can also play the full game

Mixtures can be easily separated. Methods like sieving, filtering, chromatography and evaporating can be used.

A mixture made of solid particles of different sizes, for example sand and gravel, can be separated by sieving.

Separating a mixture of iron filings and sand

A mixture of iron filings and sand can easily be separated using a magnet. The iron filings are attracted to the magnet, but the sand is not. A mixture of iron filings and sulfur powder could be separated in exactly the same way.

Remember

Filtration is a method for separating an insoluble solid from a liquid. When a mixture of sand and water is filtered:

• the sand stays behind in the filter paper (it becomes the residue)
• the water passes through the filter paper (it becomes the filtrate)

The slideshow shows how filtration works:

Evaporation is used to separate a soluble solid (i.e. a solid that dissolves) from a liquid. For example, copper sulfate is soluble in water – its crystals dissolve in water to form copper sulfate solution.

During evaporation, the water evaporates away leaving solid copper sulfate crystals behind.

Simple distillation is a method for separating the solvent from a solution. For example, water can be separated from salt solution by simple distillation.This method works because water has a much lower boiling point than salt. When the solution is heated, the water evaporates. It is then cooled and condensed into a separate container. The salt does not evaporate and so it stays behind.

Every pure substance has its own particular melting point and boiling point. One way to check the purity of the separated liquid is to measure its boiling point. For example, pure water boils at 100°C. If it contains any dissolved solids, like salt, its boiling point will be higher than 100°C. That is why salt is often added to water when cooking. The water boils at a higher temperature and so the food cooks more quickly.

Fractional distillation is a method for separating a liquid from a mixture of two or more liquids. For example, liquid ethanol can be separated from a mixture of ethanol and water by fractional distillation. This method works because the liquids in the mixture have different boiling points.

When the mixture is heated, the liquid with the lowest boiling point boils first. The vapour condenses in the condenser before the other liquid boils. The long fractionating column ensures that the second liquid does not get into the condenser until most of the first one has been removed.

One way to check the purity of the separated liquids is to measure their boiling points. For example, pure ethanol boils at 78°C and pure water boils at 100°C.

Paper chromatography is a method for separating dissolved substances from one another. It is often used when the dissolved substances are coloured, such as inks, food colourings and plant dyes. For instance, ink is a solution of dyes dissolved in a solvent of water or oil.

It works because some of the coloured substances dissolve in the solvent better than others, so they travel further up the paper.

In this case the yellow substance is the most soluble as it travelled furthest up the paper.

A pure substance will only produce one spot on the chromatogram during paper chromatography.

Two substances will be the same if they produce the same colour of spot, and their spots travel the same distance up the paper.

In the example below, red, blue and yellow are three pure substances. The sample on the left is a mixture of all three.

Chemistry is about what matter is like and how substances behave and change. A substance can be changed by heating it, adding water to it, mixing it, cooling it, and so on. The change that takes place will be a chemical change or a physical change

Changes of state are examples of physical changes.No new substances are made, and the change is often easily reversed. For example:

• liquid water can be cooled down until it forms solid water (ice)
• ice can be heated until it forms liquid water again
• water vapour condenses to water
• iodine sublimises to purple vapour

Sugar dissolved in water is another example of a physical change. You can easily separate the two by distillation.

Chemical changes are different from physical changes. New substances are made in chemical changes, and the change is often not easily reversed.

When a chemical reaction occurs, the change is called a chemical change. These include:

• methane burning in air, forming carbon dioxide and water
• iron becoming rust

This diagram shows what happens to iron and sulfur particles in a chemical reaction to form iron sulfide:

To begin with, iron particles are only joined to other iron particles, and sulfur particles are only joined to other sulfur particles. After the chemical change, iron particles and sulfur particles are joined to each other.

Chemical reactions

Atoms are rearranged in a chemical reaction. The substances that:

• react together are called the reactants
• are formed in the reaction are called the products

No atoms are created or destroyed in a chemical reaction. This means that the total mass of the reactants is the same as the total mass of the products. We say that mass is conserved in a chemical reaction.

Making iron sulfide

The reaction between iron and sulfur is often used to study elements and compounds. Iron sulfide is the compound produced in the reaction. The slideshow shows what happens in this reaction:

Iron sulfide, the compound formed in the reaction, has different properties to the elements that it is made from. The table compares the properties of iron, sulfur and iron sulfide:

The atoms in a compound are chemically joined together by strong forces called bonds. This is why the properties of a compound are different from the elements it contains, and why you can only separate its elements using another chemical reaction. Separation methods like filtration and distillation will not do this.

How to spot chemical change

• A new chemical substance is formed which usually looks different from the starting substances e.g., set light to a mixture of oxygen, a colourless gas and hydrogen, a colourless gas and a chemical reaction occurs producing water. The new substance produced, water, is very different to both oxygen and hydrogen

• Energy is taken in or given out e.g., when iron reacts with sulfur, the reaction gives out heat energy once it begins – it glows red hot

• There is fizzing or sign of a new gas

• There is a permanent colour change

• The change is usually difficult to reverse

Summary

Physical change

• no new substances are made
• no chemical reaction occurs
• easily reversed
• e.g. change in state (solid, liquid, gas) like ice melting to water or making a mixture

Chemical change

• a new substance is always formed
• a chemical reaction occurs
• there may be fizzing or signs of a new gas
• heat or light produced
• there may be permanent colour change
• there may be loss of magnetism
• e.g., heating a mixture of iron and sulfur to produce iron sulfide

The changes in chemical reactions can be expressed using equations.An equation is normally written in the form:

reactants → products

The reactants are shown on the left of the arrow, and the products are shown on the right of the arrow.Do not write an equals sign instead of an arrow. If there is more than one reactant or product, they are separated by a plus sign.

Word equations

A word equation shows the names of each substance involved in a reaction and does not include chemical symbols or formulae.For example:

copper + oxygen → copper oxide

The arrow means 'reacts to make'.In this reaction, copper and oxygen are the reactants, that react to make copper oxide, the product.

When hydrogen and oxygen undergo a chemical reaction the product is water. The balanced equation is:

Two molecules of hydrogen and one molecule of oxygen react to produce two molecules of water.

More word equations

Here are some more examples:

sodium + chlorine → sodium chloride

Notice: the non-metal (chlorine) changes its ending to ‘ide’

calcium + oxygen → calcium oxide

sodium + sulfur → sodium sulfide

magnesium + bromine → magnesium bromide

calcium carbonate → calcium oxide + carbon dioxide

Potassium hydroxide reacts with sulfuric acid. Potassium sulfate and water are formed in the reaction.

The word equation is:

potassium hydroxide + sulfuric acid → potassium sulfate + water