The pH scale

Home learning focus

Learn about universal indicator and the pH scale.

This lesson includes:

  • one video explaining universal indicator and how its used
  • one activity

Learn

Solutions can be either acidic, alkaline or neutral:

  • we get an acidic solution when an acid is dissolved in water
  • we get an alkaline solution when an alkali (or base) is dissolved in water
  • solutions that are neither acidic nor alkaline are neutral

Pure water is neutral, and so is petrol.

Universal indicator

Universal indicator doesn't just tell us if something is an acid or alkali, it also tells us how strongly acidic or alkaline something is.

When universal indicator is added to a solution, it will turn a specific colour of the rainbow.

This spectrum is called the pH scale and it gives a number, from 0 to 14, to each of the different colours.

  • pH 0 (bright red) is a strong acid
  • pH 7 (green) is neutral
  • pH 14 (bright purple) is a strong alkali
The pH scale

Watch this video where Jon Chase explains the pH scale in more detail and shows how universal indicator can be used to monitor the pH of the oceans.

Using pH indicators

Neutralisation

When you mix together an acid and an alkali, a chemical reaction takes place. The reaction is called neutralisation.

A neutral solution is made if you add just the right amount of acid and alkali together.

Neutralisation is an exothermic reaction, so the reaction mixture warms up during the reaction.

Using neutralisation

Here are some ways neutralisation is used:

  • farmers use lime (calcium oxide) to neutralise acid soils
  • your stomach contains hydrochloric acid, and too much of this causes indigestion. Antacid tablets contain bases such as magnesium hydroxide and magnesium carbonate to neutralise the extra acid
  • bee stings are acidic. They can be neutralised using baking powder, which contains sodium hydrogen carbonate

Practise

There are lots of ways to try out your science skills.

Activity 1

Make your own pH scale

Make your own pH scale with this downloadable cut and stick worksheet from Beyond.

You could use a pair of scissors and glue, or simply draw your answers out on a piece of paper.

Make your own pH scale

There's more to learn

Have a look at these other resources around the BBC and the web.

KS3 Chemistry
11-14 Chemistry
Bitesize Daily lessons