What is a salt?
A salt is an ionic compound formed when the hydrogen ion, H+, of an acid is replaced by a metal ion or the ammonium ion, NH4+. For example, in sodium chloride NaCl the H+ of HCl is replaced by Na+.
| Acid | Salt formed | Example |
|---|---|---|
| Hydrochloric acid | Chloride | NaCl |
| Sulphuric acid | Sulphate | CuSO4 |
| Nitric acid | Nitrate | KNO3 |
| Carbonic acid | Carbonate | CaCO3 |
Crystals and solubility
Many salts form crystals — solids with a regular geometric shape, flat faces, sharp edges and fixed angles between faces. Crystals of the same substance always have the same shape.
Key idea / 要点
Soluble: all nitrates; all sodium, potassium and ammonium salts; most chlorides and sulphates.
Insoluble: most carbonates; AgCl, PbCl2; BaSO4, PbSO4, CaSO4.
Worked example
Which salt is formed when nitric acid reacts with potassium hydroxide?
Answer: Potassium nitrate, KNO3. The H+ of HNO3 is replaced by K+: HNO3 + KOH → KNO3 + H2O.
Uses in daily life: sodium chloride NaCl flavours and preserves food; sodium hydrogencarbonate NaHCO3 is baking soda; calcium carbonate CaCO3 is used in cement and toothpaste; ammonium salts are fertilisers; hydrated copper(II) sulphate CuSO4·5H2O is a blue crystal used as a fungicide.
The solubility rules are worth memorising because they decide how a salt can be prepared and whether mixing two solutions will form a precipitate. Knowing, for example, that all nitrates and all sodium salts are soluble but that most carbonates and silver halides are not, lets you predict the outcome of a reaction before carrying out the experiment.
Remember
- Salt = acid with its H+ replaced by a metal or NH4+ ion.
- All Na+, K+ and NH4+ salts and all nitrates are soluble.
- Crystals have fixed shapes and fixed angles.