For your Key Stage 3 science exams you should know that acids have a pH of less than 7. The pH scale is a measure of how concentrated the hydrogen ions are in a solution. Acids with a low pH (nearest to 1) are very acid. Your stomach makes hydrochloric acid: the pH in your stomach should be about pH 2. It has to be very acid to digest the protein in your food because pepsin (the enzyme in your stomach) works best at pH 2.

Vinegar is also an acid, but it is a very weak acid. Do you put acid on your food? Probably.

The pH scale goes from 1 to 14. Any chemical which has a pH above 7 is the opposite of an acid, we call these chemicals alkalis. Any chemical which releases hydrogen ions when dissolved in water is said to be an acid. Find out about alkalis which are the opposite of acids.

Find out about indicators and how to make your own “litmus paper”.

I hope that you will be able to remember most of the facts on this page, but you must memorise the definition of acids. Write it on a small index card. Put the red words on one side of the card and the blue ones on the other. Add the card  to your revision pack.

Acids are chemicals which release hydrogen ions when dissolved in water. They have a pH of less than SEVEN.

Find out more about the pH scale from my GCSE page.

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