Year 8 Science: Separating Mixtures, Filtration and Chromatography

Year 8 is a good time for pupils to make the link between what a mixture is and how its ingredients can be separated. In England, the KS3 science National Curriculum expects pupils to learn about mixtures, including how to separate them using techniques such as filtration and chromatography. These ideas are not just classroom theory: they help children explain everyday processes, interpret experimental results and prepare for the more demanding chemistry they will meet at GCSE.

At this stage, pupils should be learning to choose a separation method by considering the properties of the substances involved. They should also practise describing methods accurately, recording observations and drawing conclusions from evidence. A little well-timed practice at home or in school can help make unfamiliar scientific vocabulary feel much more manageable.

What is a mixture?

A mixture contains two or more substances that are together but not chemically joined. Because the substances have not formed a new chemical compound, they can often be separated using physical methods. Examples include sand mixed with water, salt dissolved in water, or the different coloured substances in ink.

Pupils sometimes confuse a mixture with a compound. A useful distinction is that the substances in a mixture retain their own properties and can usually be separated by physical processes. In a compound, elements are chemically bonded and cannot be separated by simple physical methods. Year 8 pupils do not need to master every detail of chemical bonding, but they should be able to use this distinction when explaining why a particular separation technique works.

Encourage children to identify what is present before choosing a method. Is one substance a solid and the other a liquid? Is a solid dissolved, or can you see its particles? Are two liquids mixed together? These questions help pupils reason rather than simply memorise a list of techniques.

Filtration: separating an insoluble solid

Filtration is used to separate an insoluble solid from a liquid. “Insoluble” means that the solid does not dissolve in the liquid. If sand is mixed with water, for example, the sand remains as solid particles. Pouring the mixture through filter paper allows the water to pass through while the sand is trapped.

The liquid that passes through the filter is called the filtrate. The solid left behind is the residue. These terms are important in KS3 science and often appear in questions about practical methods. A useful way to remember them is to picture the filtrate as the liquid that has filtered through.

For a safe classroom or supervised home demonstration, use clean sand and water, a funnel and filter paper. Fold the paper as instructed, place it in the funnel, and pour the mixture slowly. Pupils can observe that the water collects below while the sand remains on the paper. They should understand that filtration does not remove a dissolved substance. If salt has dissolved in water, the salt particles pass through the filter along with the water.

A common misconception is that filter paper acts like a sieve for anything unwanted. In fact, filtration only separates particles that are large enough to be trapped. Dissolved particles are too small to be retained by ordinary filter paper. Asking pupils to predict whether sand, salt or both will remain on the paper is a simple way to check their understanding.

Chromatography: separating dissolved substances

Chromatography can separate substances that are dissolved in a solvent, such as the different dyes in some inks. In paper chromatography, a spot of ink is placed near the bottom of a strip of paper. The bottom edge is then placed in a suitable solvent, with the ink spot kept above the solvent level. As the solvent travels up the paper, it carries the dyes with it. Different dyes move at different speeds and travel different distances, so they may separate into coloured spots.

Pupils should learn to connect the result to the properties of the substances. A dye’s movement depends on how well it dissolves in the solvent and how strongly it is attracted to the paper. A dye that is more soluble in the solvent may travel further, while one that is more attracted to the paper may move less. At Year 8, it is enough for pupils to describe these differences clearly and use their observations as evidence.

Chromatography is also a useful opportunity to practise careful experimental technique. Use pencil, not ink, to draw the baseline, because pencil does not dissolve and run up the paper. Keep the sample spots small, place the solvent below the baseline, and avoid disturbing the paper while the solvent rises. When the solvent is close to the top, remove the paper and mark the solvent front promptly. Only use solvents approved by the school or a responsible adult; pupils should not experiment with household chemicals unsupervised.

Ask children to compare the spots produced by different pens or food colourings. If two samples produce the same pattern under the same conditions, they may contain similar dyes, but this does not prove that they are identical. This is a valuable chance to discuss the limits of evidence as well as the results themselves.

Choosing the right separation method

A simple decision-making routine can help pupils select a suitable method. First, identify the substances in the mixture. Then consider whether a solid is insoluble, whether a substance is dissolved, and whether the components have different physical properties that can be used to separate them.

Filtration is appropriate for an insoluble solid mixed with a liquid. It will not separate dissolved salt from water. Chromatography can separate some dissolved substances, particularly coloured mixtures such as inks. Other separation methods build on the same reasoning. Evaporation can recover a dissolved solid from a solution, while sieving can separate solids of different particle sizes. Year 8 teaching may introduce several of these methods, so pupils benefit from explaining not only what to do but why it is suitable.

For example, if asked to separate sand and salt from a mixture, pupils should recognise that adding water dissolves the salt but not the sand. Filtration can then remove the sand, and evaporation can recover the salt from the salt solution. This multi-step example encourages pupils to plan a method in sequence and use scientific vocabulary accurately.

Building confidence with practical work and revision

Practical science is most useful when pupils make predictions, observe carefully and explain what their results show. Before an experiment, ask your child to draw or describe what they expect to happen. During the activity, encourage precise observations, such as “the black ink separated into blue and yellow spots”, rather than vague comments such as “it changed”. Afterwards, ask which evidence supports their conclusion.

Worksheets can reinforce these practical experiences when they include diagrams to label, method-ordering questions and short explanations. A good revision routine is to revisit the key terms—mixture, soluble, insoluble, filtrate, residue and solvent—and use each in a sentence. Pupils can also practise matching a mixture to a method and explaining why alternatives would not work. Short, regular practice is often more effective than trying to learn every term in one session.

When supporting a child, keep questions open and manageable. “What is dissolved here?” or “Which part will pass through the filter?” invites them to reason aloud. If they make a mistake, ask them to use the evidence from the practical to reconsider their answer. This builds confidence and helps them see science as a process of testing ideas, not just recalling facts.

A practical conclusion

Separation techniques become easier when pupils focus on the properties of the substances in a mixture. In Year 8, children should be able to explain that filtration removes an insoluble solid from a liquid, while chromatography can separate substances such as dyes dissolved in a solvent. With safe practical demonstrations, clear vocabulary and short, regular worksheets for practice and revision, parents and teachers can help pupils meet KS3 expectations and develop the careful reasoning that underpins further science learning.