In this module, you will learn:

  • about indicators such as universal indicator
  • about the pH scale

Resources to help you:

  • Textbook: page 118, 119 and 120
  • Notebook: page 31

Are you weak or strong?

Sometimes, we need to know if an acid or an alkali is weak or strong. It can be an important safety measure when handling acids and alkalis.


To help us find the strength, we use an indicator to tell us their pH value.

The pH value (which stands for potential of hydrogen) ranges from 0 to 14.

A pH value of 0 to 2 indicates it to be a strong acid.

A pH value of 3 to 6 is considered as a weak acid.

A pH value of 7 is considered as neutral.

A pH value of 8 to 11 is a weak alkali.

A pH value of 12 to 14 is a strong alkali.

The different colours of universal indicator corresponding with different pH values

Credit: @Dejan Jovic DJ via Wikipedia [CC BY-SA 4]

Watch the video

Learn about different indicator to determine pH value

Source: https://www.youtube.com/watch?v=xYQlvTblgCY

You can change the speed to 0.75x to make it easier to listen.

Watch the video

Learn how to make your own indicator at home

Source: https://www.youtube.com/watch?v=I18K2upEHLc

You can change the speed to 0.75x to make it easier to listen.

Ready for the next module?

In the next module, we will look at what happens when we add an acid to an alkali.

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