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Science

Acids and bases

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The pH scale measures how acidic or alkaline a substance is. Substances with a pH of 7 are neutral, substances with a pH greater than 7 are alkaline (or 'basic') and substances with a pH lower than 7 are acidic. Alkalis are 'bases' that are soluble in water. (All alkalis are bases but not all bases are alkalis.)

Acids and alkalis

The pH scale

The chemical properties of many solutions enable them to be divided into three categories – acids, alkalis and neutral solutions. The strength of the acidity or alkalinity is expressed by the pH scale.

  • Solutions with a pH of 7 are neutral.

  • Solutions with a pH greater than 7 are alkaline. The higher the pH, the stronger the alkali.

  • Solutions with a pH less than 7 are acidic. The lower the pH, the stronger the acid.

If universal indicator is added to a solution it changes to a colour that shows the pH of the solution.

Diagram of the PH scale

Diagram of pH scale and universal indicator colours

Here are some examples of acids, alkalis and their pH values.

The pH scale

pHExample
strong alkali14drain cleaner
13bleach
12soapy water
11ammonia
10milk of magnesia
9baking powder
8sea water
neutral7pure water
6saliva
5black coffee
4acid rain
3vinegar
2lemon juice
1stomach acid
strong acid0battery acid

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