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Understand the pH Scale

In this worksheet, students will receive an introduction to acids and alkalis and the use of the pH scale to identify a substance as being acidic, alkaline or neutral and indicating the relative strength of the acid or alkali.

'Understand the pH Scale' worksheet

Key stage:  KS 3

Year:  Year 7 Science worksheets

Curriculum topic:   Chemistry: Chemical Reactions

Curriculum subtopic:   pH Scale

Difficulty level:  

Worksheet Overview

Acids have a reputation for being nasty chemicals that burn through floors and metal structures with great ease. But did you realise that vinegar is an acid? This activity tackles some of the myths associated with acids and their chemical opposites - alkalis. Many of the products you find at home are acidic or alkaline.


Having vinegar on your chips is not dangerous, but drinking a lab acid like hydrochloric acid could be fatal.  This is because hydrochloric acid is a much stronger acid than vinegar

The strength of an acid or alkali can be measured using the pH scale.


pH Scale

The pH scale is a measure of how strong or weak an acid or alkali is. It is a continuous scale from below 0 to above 14. The colour chart below shows the colour that universal indicator turns when it’s mixed with solutions of different pHs:


pH scale


Neutral solutions such as water have a pH of 7.


Acids have a pH of less than 7. The lower the pH number of an acid the stronger the acid is.  So an acid like hydrochloric acid with a pH of 1 is a lot stronger than vinegar with a pH of 3.


Alkalis have a pH of greater than 7. The higher the pH number of an alkali the stronger it is. So an alkali like sodium hydroxide with a pH of 14 is a lot stronger than baking soda with a pH of 9. 


Acids and alkalis can cancel each other out, they can form neutral solutions. This is called neutralisation. 


Farmers who have soil that is too acidic to grow crops, for example, add lime which is an alkali to the soil. This makes the soil more neutral and occurs when any acid or alkali are mixed together.


Sometimes we can suffer indigestion from excess amounts of stomach acid. Taking an alkaline substance such an indigestion tablet can neutralise the excess acid and sooth the burning pains of indigestion.


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