Science at key stage 3    (Year 8)

• Review and extend pupils' understanding of the difference between elements and compounds or non-elements by providing them with a series of cards, the first giving a description of appearance, the second the formula and the third the composition, eg one atom of oxygen joined to two atoms of hydrogen, and asking pupils in groups to match them in sets of three and to sort into elements and non-elements.
• Make sure that pupils are clear that compounds contain atoms of more than one kind joined together.

• Demonstrate making or invite pupils to make a mixture from two elements: sulphur (powder) and iron (powder). Ask pupils to heat a mixture of iron and sulphur in ignition tubes until they observe a red glow and, after their tube has cooled, to extract the contents and try to find out if it still contains a mixture or if a new chemical (compound) has been made. Through discussion of their results, establish that they have made a compound. Help pupils to write picture and word equations for the reaction.
• Demonstrate that the compound iron sulphide behaves differently from its constituent elements sulphur and iron, eg by adding a small amount of dilute acid to both and observing the differences in the way the mixture behaves compared to the compound.
• Ask pupils for names of compounds they have used, eg water, carbon dioxide, copper carbonate; provide them with samples and ask them to compare the compounds with the elements from which they are made. Establish, through discussion of the formulae of some of the oxides made in unit 8E 'Atoms and elements', eg magnesium, sodium and aluminium oxides, that compounds are made from elements in fixed proportions. Provide pupils with drawings or software simulations showing particles in examples of elements, compounds and various mixtures of elements and/or compounds. Ask them to identify the types of particles present and what the drawings represent in terms of elements, compounds and mixtures.

• Ask pupils to carry out a number of test tube chemical reactions in which visible changes occur, eg
• mixing sodium carbonate solution and iron (II) chloride solution
• adding dilute hydrochloric acid to solid magnesium carbonate
• adding dilute ammonia solution to copper sulphate solution
• heating sucrose
• Ask pupils to record their observations carefully, telling them that they are looking for evidence that chemical reactions making new materials have taken place. Ask pupils, in groups, to agree on a short paragraph about the reactions setting out the observations and evidence, then to exchange work and decide whether they agree or disagree with the evidence suggested, explaining their reasons.

• Ask pupils to produce a concept map related to chemical change using terms, eg atom, element, compound, burning, oxygen, formula, symbol, chemical, reaction, copper, oxide. Discuss pupils' maps with them and if necessary help them to make an exemplar map to refer to in later work.

• Remind pupils of work about mixtures in unit 7H 'Solutions' and show them examples of mixtures they may have encountered, eg iron and sulphur, chalk in water, seawater, air, soil, rocks, ink, shaving foam. Elicit what they understand by the term 'mixture', eg by asking them to draw diagrams to illustrate the particles in elements and compounds and extending this to mixtures, and establish their criteria for deciding whether something is a mixture or not. Introduce the idea that compounds can be represented by a formula whereas mixtures vary in composition, eg by showing samples of compounds together with molecule models, and contrasting these with samples of mineral water together with labels from bottles showing that the water is a mixture.
• Ask pupils for their ideas of the meaning of 'pure' when applied to a material, eg What is 'pure' orange juice? What is 'pure' water? Establish what is meant by 'pure' and 'impure' and link back to the illustrations of elements, compounds and mixtures made earlier in the activity. Reinforce using simulation software illustrating elements, compounds and mixtures.

• Ask pupils how a mixture of a solid and liquid, eg blue ink, or of coloured dyes, eg inks in a felt-tip pen can be separated. Use secondary sources, eg video clips, to illustrate that air is a mixture and ask them why it might be important to separate the gases in air. Provide pupils with appropriate information about the separation of air into its components and the uses of these. Extend the work, as appropriate, by asking them to find out differences between inhaled and exhaled air, the importance of ventilation in rooms, how the composition of air dissolved in water varies and about the composition of air in passenger planes. Ask pupils to produce an information leaflet about air reminding them about the importance of organising facts, ideas and information into sentences, which are then grouped into paragraphs with appropriate linkages.

• Ask pupils to look back at the work they did on elements in unit 8E 'Atoms and elements' and to identify boiling points and melting points of some of these elements. Illustrate with video clips the very high temperatures at which some metals melt. Establish, through discussion of their data and other data from secondary sources, that changes of state of pure elements and compounds occur at a fixed temperature and that the temperature is a characteristic of the material that is changing state.
• Give pupils examples of melting points and boiling points. Ask them whether boiling point is always higher than melting point and help them to use database software to test their predictions. Establish through discussion that melting and freezing are opposites and occur at the same temperature for a given material. Illustrate with video clips that gases can be cooled enough to liquefy and may eventually solidify. Relate this back to work on separation of air.
• Ask pupils if they can find in any data book or database the boiling point of air and to explain why they cannot. Carry out a quick demonstration of the differences in the boiling point of tap water and salt solution and introduce the idea that mixtures do not have fixed melting or boiling points.

• Provide pupils with two liquids labelled A and B (distilled water and salt solution). Tell them that they are going to investigate how temperature changes as they cool (surrounded by an ice/salt mixture) and use this and any other sample tests to find out as much as possible about the liquid. Ask the pupils to plan what to do, including how frequently they will make measurements, and to produce an account of what they did, tables and graphs of results and to use all their results to draw conclusions about the liquids.

• Give pupils a list of statements, eg
• is made up of atoms
• has a definite composition
• is a result of atoms joining together
• contains different substances which are not chemically combined
• can be represented by a chemical formula
Ask them to assign each to one or more of element, compound and mixture.
• Some pupils will also be able to sort the statements in each category so that similar ones are grouped together. Check those that apply to two or to all three, eg contains molecules, and those which are 'difficult', and agree a summary sheet consisting of correctly allocated statements. Ask them to add familiar examples of elements, compounds and mixtures to their summary sheet.