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Science at key stage 3    (Year 7)

Unit 7H: Solutions
Section 1: How can we tell whether a liquid is a mixture?

QCA

Objectives
Children should learn:
  • that some solids dissolve in liquids and others do not
  • that many common materials are mixtures
  • that mixtures can be separated

Activities

Outcomes
Children:
  • 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'.
  • name some solids that dissolve in water and some that do not
  • identify the components of some mixtures, eg seawater is water with salt and other solids dissolved in it
  • describe one way of separating the components of a mixture

Points to note
  • This activity is designed to find out what pupils already know about solid/liquid separation. In the light of this, teachers may wish to put more emphasis on making mixtures and separation techniques with some pupils and on particle explanations with others.
  • Extension: pupils could investigate the labels on packaging of household items and food to find out which are mixtures and which are not.

    • Safety

    • teachers will need to check pupils' plans for health and safety before practical work begins. Ethanol is highly flammable, so there should be no naked flames in the room if ethanol is used. Copper sulphate solution is harmful if the concentration is greater than 1.0 mol dm-3. Eye protection should be worn

Sections in this unit

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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.
1. How can we tell whether a liquid is a mixture?
2. How much salt can we get from rock salt?
3. What happens to the solute when a solution is made?
4. How can we separate solvents from solutes?
5. a. How can chromatography separate and identify substances in mixtures?
6. b. How can chromatography separate and identify substances in mixtures?
7. Checking progress
8. Is there a limit to the amount of solid that will dissolve in a liquid?
9. What else affects solubility?
10. Reviewing work