Science at key stage 3 (Year 8)
Unit 8F: Compounds and mixtures
Section 7: c. Are there other sorts of material besides elements and compounds?
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Objectives |
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- that elements and compounds melt and boil at particular temperatures
- that mixtures do not melt or boil at fixed temperatures
- to use data from secondary sources
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Activities |
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Outcomes |
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Children: |
- 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.
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- identify the melting and boiling points of a range of elements and compounds
- explain that these are characteristic of the element or compound
- describe how the melting point or boiling point of a mixture varies with composition
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Points to note |
- Pupils will have constructed a temperature line in unit 8I 'Heating and cooling'. They could add some of the melting points and boiling points considered in this activity. At this stage it is not necessary to consider variation of boiling point with atmospheric pressure unless pupils raise the question.
- If it is possible to obtain a sample of liquid nitrogen, demonstration of its properties provides additional interest and challenges pupils' thinking.
Safety
- ensure liquid nitrogen is transported safely in a properly sealed Dewar flask. Beware cold burns. Obtain a special risk assessment
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This unit is divided into sections. Each section contains a sequence of
activities with related objectives and outcomes. You can view this unit by
moving through the sections or print/download the whole unit.
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