Science at key stage 3    (Year 7)

• Present pupils with a selection of liquids, eg distilled water or deionised water, seawater, a suspension of chalk in water, ethanol, copper sulphate solution, and ask pupils about them, eg Is it water? Is it pure? Ask pupils to describe work they did on solutions and on separating solids from liquids in key stage 2. Challenge them to devise techniques, eg filter, evaporate to dryness, to find out whether the liquid is a mixture or not. Ask them to record their observations and explain what their method showed. Help the class to summarise different approaches used and introduce the terms 'soluble', 'insoluble' and 'solute'.

• Present pupils with a sample of rock salt and say that pure salt can be obtained from this. Ask them to plan a way of obtaining a sample of pure salt. Extend this for some pupils by asking them how they could find out how much of their rock salt is pure salt and to compare their results with those of others. Ask pupils to produce an account of what they did, describing and explaining each stage. Emphasise that, although the salt dissolves, it doesn't disappear.
• Show a video about the occurrence, extraction and uses of salt. Relate this to the processes pupils have used in extracting their sample.

• Show pupils a beaker of water of a particular mass, eg 100g, and ask them to predict what the total mass will be if some salt, eg 4g, is dissolved in the water, and to test their predictions by weighing. Ask them to explain why the mass remains the same and to say how much salt they would expect to get if they evaporated the water.
• Remind pupils of work they did on particles in solids and liquids in unit 7G 'Particle model of solids, liquids and gases', referring back to the annotated diagrams drawn at the end of the unit.
• Show a model or models, eg ICT simulation, mixing rice and peas, to illustrate the mixing of particles when a solid dissolves in a liquid, asking pupils to put in their own words what is happening.
• Ask pupils to draw the water particles before and after the addition of salt and to use these to explain, eg that mass is conserved, why filtering will not separate the salt.

• Explain to pupils that work so far has concentrated on the solute in a mixture but that it is often very important to separate and collect the liquid, eg in purifying water.
• Remind pupils of work done on evaporation and condensation at key stage 2, eg by showing them a sample of blue ink and asking them to predict the colour of the water obtained when it was evaporated and then condensed. If necessary, carry out a quick demonstration. Ask them how they could obtain drinkable water from seawater, eg in an area where there is a lot of seawater, but no fresh water. Discuss pupils' ideas with them and introduce the term 'distillation'. Help pupils to test their ideas by carrying out a simple distillation process.
• Ask pupils to explain the process, eg by using a flow chart, annotated drawing.

• Remind pupils of work they did earlier on showing whether a liquid was pure or not. Explain that they are going to explore some other techniques for this.
• Demonstrate how to separate the different coloured compounds in an ink mixture on blotting or filter paper, using a wick of the paper dipped into the solvent (water). Discuss and establish with pupils why different coloured ink solids travel different distances, perhaps using the analogy of the solvent particles giving the solute particles 'piggyback rides', so those which attach more firmly to the solvent particles can be carried further than those which do not, in a given amount of time.

• Ask pupils to use chromatography to compare the components of dyes, eg the colouring on sugar-coated chocolate sweets, to find out whether different colours include the same dyes and to explain what they found, eg using drawings and annotated diagrams. Extend by providing pupils with prepared chromatograms and information about the contexts in which these might be needed, eg in forensic science, in identifying traces of substances in urine or medical preparations, and asking them to interpret the evidence from each chromatogram.

• Use ICT simulation to show pupils how to model a change, eg some solid dissolving in water, by representing the particles themselves, making sure that those representing the solid are clearly distinguishable from those representing the water. Ask groups of pupils to work out how to model other changes, eg adding an insoluble solid to water, the formation of a suspension, separation by chromatography of two solutes, the evaporation of a solution. Help pupils to write a brief description of each change.

• Present some pupils with a selection of solids, eg salt, bicarbonate of soda, potassium nitrate, and ask them how they could find out whether there is a limit to how much will dissolve in water at room temperature. Ask other pupils to find out whether there is a limit to the amount of solid, eg salt, that will dissolve in different liquids, eg water, ethanol. Discuss with them how they will decide when no more dissolves and suggest that different groups use different volumes of solvent. Bring together results for the same solvent and look for patterns in these. Introduce the terms 'saturated solution' and 'solubility'.

• Ask pupils to suggest what else affects how much solid dissolves in a liquid, perhaps showing them a saturated solution, eg of benzoic acid, which forms crystals as it cools. Provide pupils with a table of data showing approximate solubilities of solutes at different temperatures. Help them to work out how much they would need to dissolve in 10g of water to make a saturated solution at a particular temperature. Ask pupils to prepare a warmed solution with a suitable amount of solid and to cool it down to identify the temperature at which crystals appear. Put together class results and help pupils to draw a graph showing how solubility varies with temperature and to describe what the graph shows and what the solubility might be at other temperatures.

• Provide pupils in groups with two sets of cards: each card in one set has the name of a change or technique, each card in the other set has the explanation of a change or technique. Ask them to match the correct explanation to the change or technique. For some pupils increase the range of changes, including some incorrect explanations to encourage discussion and deeper thinking about the phenomena.
• Ask pupils to work in groups and list the key ideas about the changes, including two examples of each. Ask them for their ideas and discuss the changes, emphasising that those encountered in this unit can be relatively easily reversed. Together with pupils agree a summary of the unit.