Science at key stage 3    (Year 7)

• Check that pupils are familiar with the terms 'solid', 'liquid' and 'gas' and have some understanding of what these mean. Present them with examples and ask them to allocate each example to the appropriate class.
• Introduce the idea that pupils will work in groups to carry out some quick experiments which will help them explain why solids, liquids and gases behave differently, eg
• comparing the masses of identically-sized blocks of two or three different materials such as wood, glass and metal
• putting one small coloured crystal into a beaker of cold water and one into a beaker of hot water
• trying to fit a metal bar into a gauge before and after strong heating
• opening a perfume bottle at arm's length
• heating one end of a metal rod that has paperclips attached by petroleum jelly
• trying to depress the plungers of three sealed syringes, one containing a solid, one a liquid and one a gas
• adding small weights, one at a time, to a thin wire suspended from a clamp until it snaps
• Ask pupils to discuss with others what they observe and try to explain what has happened.
• If pupils cannot give any explanations to start with, it may be helpful to suggest that they try to imagine they can see what is happening through powerful microscopes.
• Ask groups of pupils to present observations and to describe and explain them, eg through oral, flip-chart or overhead-projector (OHP) presentation. Discuss different ideas with pupils and ask them to compare their own ideas with those of others.

• Introduce the idea that scientists collect data or evidence and that they try to think creatively to explain this evidence. Theories result from scientists relating their ideas to the evidence and refining their ideas.
• Present pupils with a game or puzzle to play, eg a murder mystery game, where each group is given some evidence about an event, eg a fictitious murder, and is asked to establish what happened on the basis of the evidence.
• Provide further evidence, which could be given part-way through, to encourage pupils to modify their original suggestion on the basis of the new evidence. Ask pupils to reflect on their ideas, to explain why they arrived at their first conclusion, and what caused them to change their ideas.

• Give pupils a range of materials to classify as solid, liquid or gas. Include some materials that pupils find difficult to classify, eg paper, sand, jelly, talc, toothpaste, tomato sauce, reusable adhesive, etc. Tell pupils they should be prepared to justify their classification, making explicit the criteria they used. Ask them to list the properties of solids, liquids and gases and use this to develop a key for classifying materials.
• Ask pupils to imagine they can see what the materials are made from by using an immensely powerful microscope and, in groups, to discuss and create their own theory or model of what the materials are made from. Invite pupils to communicate these to the class. At this stage, help pupils identify inconsistencies between theory and evidence, but do not give the correct particle theory.

• Using a variety of media, eg simulation software, three-dimensional models, diagrams, texts, modelling using pupils, make explicit to pupils the accepted theory about particles in terms of their proximity and motion.
• Through discussion and questioning, establish the similarities and differences between their theories and the accepted one and show how the accepted one explains the evidence.

• Invite pupils to carry out additional quick experiments in groups, eg
• placing coloured crystals on agar gel and observing the diffusion of colour
• placing distinct layers of water and ink in plastic syringes and observing the mixing of colour over a short period of time
• observing dust particles in a beam of light
• Ask them to try to explain their observations using knowledge and understanding about particles, discussing their ideas with each other. Explain that they must check that the explanations they will give to the class later include use of the particle theory.

• Give pupils access to all the experiments they had at the start of the unit and ask groups to present explanations using particle models, eg orally or using flip charts, OHPs.
• Demonstrate that gases mix by diffusion, eg by mixing bromine and air, or nitrogen dioxide and air, or allowing gas jars of hydrogen and air to mix. Use a range of methods, eg video clips, simulation software, pupils modelling, to illustrate the movement of gas particles and explain diffusion.
• Discuss with pupils the idea of air particles all around before demonstrating the collapsing can experiment. Ask pupils, in groups, to explain why the can collapses; link their ideas back to pressure of gas inside and outside the can.

• Ask pupils to make annotated drawings to describe the arrangement and movement of particles in solids, liquids and gases. Ask them to choose two of the experiments they have carried out, or have seen, and describe what they did, what they observed and to explain their observations in terms of the particle theory.