Class practical

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

Lesson organisation

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.

Apparatus Chemicals

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

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

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection. 

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, 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

a Tear each piece of litmus paper into 3 smaller pieces so that you can test at least 9 substances.

b 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.

c 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. ‘A solution of vinegar is acidic. Vinegar is an acid.’

Health & Safety checked, 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015