An acid is neutralised by an alkali or a base. An alkali is a soluble metal hydroxide, such as sodium hydroxide. A base is a substance that neutralises an acid, and includes insoluble metal hydroxides and metal oxides. Acids are also neutralised by metal carbonates. The products are always a salt and water, and a carbonate also gives off carbon dioxide.
- acid + alkali → salt + water: NaOH + HCl → NaCl + H2O
- acid + metal oxide → salt + water: CuO + H2SO4 → CuSO4 + H2O
- acid + metal carbonate → salt + water + carbon dioxide: CaCO3 + 2HCl → CaCl2 + H2O + CO2
The salt you make depends on two things. The second part of its name comes from the acid: hydrochloric acid produces chlorides, nitric acid produces nitrates and sulfuric acid produces sulfates. The first part comes from the positive ions in the base, alkali or carbonate. So sodium hydroxide and hydrochloric acid make sodium chloride, and magnesium oxide and nitric acid make magnesium nitrate.
You can use the formulae of common ions to work out the formula of a salt. The total charge on the ions in a salt must come to an overall charge of zero. Calcium ions are Ca2+ and nitrate ions are NO3-, so each calcium ion needs two nitrate ions. The salt is calcium nitrate, Ca(NO3)2.