A collection of experiments that demonstrate chemical concepts and processes.
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Acid or alkali? Acidic or alkaline?

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.

Chemicals

Access to small amounts of common everyday substances such as:

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.

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

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.

 

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


If: 

  • the blue Litmus paper turns red 
  • the red Litmus paper is unaffected 
  • then the substance is acidic.

If: 

  • the red Litmus paper turns blue 
  • the blue Litmus paper is unaffected 
  • then the substance is alkaline.

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 and Safety checked November 2006

 

Page last updated on 09 December 2011