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
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 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
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.
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.
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
There are lots of ways to try out your science skills.
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.
There's more to learn
Have a look at these other resources around the BBC and the web.