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A salt is a compound formed when some or all the hydrogen ions in an acid are replaced by metal ions or ammonium ions. For example, zinc sulfate is the salt formed when zinc ions replace the hydrogen ions of sulfuric acid.

Most group 1 (I), group 2 (II), aluminium and zinc salts are white. They produce colourless solutions when dissolved in water. Transition metal salts generally produce coloured solutions when dissolved in water.

Preparing soluble salts

A pure, dry sample of a soluble salt can be prepared in two ways.

Method one

• add excess insoluble substance (base) to warm acid
• filter off excess base
• evaporate filtrate to half volume
• cool to allow crystals to form
• filter off the crystals

Method two

• add alkali to acid, or vice versa, in the presence of an indicator until the indicator changes colour
• repeat the process without the indicator, or remove the indicator using charcoal
• evaporate the solution to half volume
• cool to allow crystals to form
• filter off the crystals

You can dry the crystals by placing it in a desiccator, in a low temperature oven, or between two sheets of filter paper.

Prescribed Practical C3 - Investigate the preparation of soluble salts