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Acids [acids: Corrosive substances which have a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions. ] have a pH of less than 7. Bases [bases: Substances with a pH higher than 7, and which have a high concentration of hydroxyl ions. Bases react with acids to form a salt and water (called neutralisation). Metal hydroxides, oxides and carbonates are all bases. ] have a pH of more than 7. When bases are dissolved in water, they are known as alkalis [alkalis: Bases which are soluble in water. ].
Salts [salts: class of chemical compounds, mostly metallic oxides. Examples are sodium chloride, potassium chloride, and magnesium sulphate ] are made when an acid reacts with a base, carbonate or metal. The name of the salt formed depends on the metal in the base and the acid used. For example, salts made using hydrochloric acid are called chlorides.
Acids and bases
You will have already learnt quite a lot 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 bases. Here are the bare bones of what you need to know:
Acids
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
Bases that dissolve in water are called alkalis. Copper oxide is not an alkali because it does not dissolve in water. 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
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.
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