Acids and bases

You'll already have learnt quite a bit about acids and bases in Key Stage 3 Science. If you feel a bit rusty on this topic, have a quick look at KS3 Bitesize Chemistry Acids and Alkalis. Here are the bare bones you need to know.

Acids turn blue litmus paper red

Alkalis turn red litmus paper blue

Acids

Hydrochloric acid

• substances with a pH of less than 7 are acids
• the stronger the acid, the lower the pH number
• acids turn blue litmus paper red
• they turn universal indicator red if they are strong, and orange or yellow if they are weak

Bases

• Substances that can react with acids and neutralise them to make a salt and water are called bases.
• They are usually metal oxides or metal hydroxides. For example, copper oxide and sodium hydroxide are bases.

Alkalis

Sodium hydroxide

• Bases that dissolve in water are called alkalis.
• Copper oxide is not an alkali because it does not dissolve in water, but sodium hydroxide is an alkali because it does dissolve in water.
• Alkaline solutions have a pH of more than 7.
• The stronger the alkali, the higher the pH number.
• Alkalis turn red litmus paper blue.
• They turn universal indicator dark blue or purple if they are strong, and blue-green if they are weak.

Neutral solutions

Distilled water

• Neutral solutions have a pH of 7.
• They do not change the colour of litmus paper, but they turn universal indicator green.
• Water is neutral.