This simple experiment involves testing a variety of substances to see if they are acidic or alkaline. Litmus paper is used as the indicator

This experiment is most suitable for introductory level chemistry and could be done as a circus with pairs/small groups of students moving from one station to another.

A discussion of the meaning of the words acid, alkali and neutral could be a good way to introduce the experiment. The idea of an indicator should be introduced as a means of distinguishing between the three possibilities, using litmus as the indicator.

Equipment

Apparatus

  • Eye protection
  • Watch glasses, 1 per sample (note 1)
  • White tiles, 1 per sample (note 1)
  • Red and blue litmus paper, 3 pieces of each per working group

Chemicals

  • Access to small amounts of common everyday substances such as (note 2)
    • fizzy drinks
    • tap water
    • de-ionised/distilled water
    • toothpaste
    • shampoo
    • soap
    • vinegar
    • lemon juice

Health, safety and technical notes

  1. Provide small amounts of the samples on watch glasses. The watch glasses can be placed on a white tile marked with the name of the substance (use a waterproof marker). The samples can be arranged around the room so that the students visit each in turn. If any solid sample is used, moisten it with a little de-ionised water.
  2. Check hazard labels on any household substances provided. If this activitiy is set as homework, especially on substances not listed above, pupils should be warned that there are many hazardous chemicals in the home. It is recommended that if chemicals such as bleach, drain clear, or scale remover are to be investigated that this is done by the teacher in school. Thick rubber gloves should be worn if planning to do this due to their hazards.

Procedure

  1. Tear each piece of litmus paper into three smaller pieces so that you can test at least nine substances.
  2. Take one small piece of red litmus paper. Dip it into one of the substances to be tested. Repeat with a small piece of the blue litmus paper.
  3. Record all observations in a suitable table. Then dispose of the pieces of litmus paper in the waste bin.

Teaching notes

The substance is acidic when:

  • the blue Litmus paper turns red and the red Litmus paper is unaffected 

The substance is alkaline when:

  • the red Litmus paper turns blue and the blue Litmus paper is unaffected 

If neither red nor the blue paper changes colour, the substance is neutral.

Students should make three lists of substances under the headings ‘acidic’, ‘alkaline’, ‘neutral’.

Encourage the students to use the terms correctly. ‘Acid’ and ‘alkali’ are nouns; the words ‘acidic’ and ‘alkaline’ are adjectives ie ‘A solution of vinegar is acidic. Vinegar is an acid.’

Primary science teaching notes

Skill development

Children will develop their working scientifically skills by:

  • Selecting and planning the most appropriate ways to answer science questions, recognising and controlling variables where necessary, including:
    • grouping and classifying things
  • Recording data and results using scientific diagrams, labels, classification keys and tables
  • Using appropriate scientific language and ideas to explain, evaluate and communicate their methods and findings

Learning outcomes

Children will:

  • Make predictions and test a variety of substances to determine if they are acidic, alkaline or neutral

Concepts supported

Children will learn:

  • Substances can be acidic, alkaline or neutral
  • Substances can be grouped according to their properties including whether they are acidic, alkaline or neutral

Suggested activity use

You could carry out this activity as a carousel of different stations, allowing the children to work in pairs or threes to make predictions and then test a range of common everyday substances to see if they are acidic or alkaline. Children can then decide how to record this information and discuss their findings as a plenary activity.

Practical considerations

Various equipment and resources are needed for this practical, including litmus paper and watch glasses.

To get the most out of the activity, children will need some simple understanding of the terms acid (noun), acidic (adjective), alkali (noun), alkaline (adjective), neutral (adjective) and indicator.

Additional information

This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology. 

© Nuffield Foundation and the Royal Society of Chemistry

Health and safety checked, 2016