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Science at key stage 3    (Year 7)

Unit 7E: Acids and alkalis
Section 3: How can acids and alkalis be identified and distinguished from each other?

QCA

Objectives
Children should learn:
  • to devise a table to show results effectively and to identify patterns in these
  • that acids and alkalis can change the colours of some dyes and that this can be used to classify them
  • the names of some common laboratory acids and alkalis
  • to classify solutions as acidic or alkaline, using indicators

Activities

Outcomes
Children:
  • Establish, by quick questioning, that pupils recall key ideas about safety.
  • Provide pupils with solutions of dyes extracted from plant material, eg red cabbage, raw beetroot, blackcurrant, litmus, and ask them to explore and record the effect of adding household and laboratory acids and alkalis to the dyes.
  • Compare the results with different dyes and establish, by comparing results from the class, that there are two classes of solution and that the dyes can indicate which is which. Introduce the terms 'indicator' and 'alkali'.
  • Provide pupils with a range of acidic and alkaline solutions and indicators and ask them to use the indicators to identify and record whether the solutions are acidic or alkaline.
  • present their results in a way which helps them identify patterns
  • recognise that solutions of dyes which show one colour in acids and another in alkalis are called indicators
  • recall the names of some common laboratory acids and alkalis
  • use their record of results to identify which solutions are acidic and which are alkaline

Points to note
  • Pupils could extract the dyes from plant materials themselves if they are familiar with the techniques required.

    • Safety

    • avoid the use of bleach, drain cleaner or descaler. 0.4 mol dm-3 laboratory acids and alkalis can be used for this activity. None are hazardous although some are irritants, and eye protection is needed. In general, alkalis are more hazardous to skin and eyes than acids of a similar concentration

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. a. What are acids and alkalis like and where do we use them?
2. b. What are acids and alkalis like and where do we use them?
3. How can acids and alkalis be identified and distinguished from each other?
4. Is there a range of acidity and alkalinity?
5. a. What happens when an acid is added to an alkali?
6. b. What happens when an acid is added to an alkali?
7. Checking progress
8. Where is neutralisation important?
9. Reviewing work