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Salts

a wide range of chemical compounds including common salt. The scientific definition of a salt is a compound formed when the hydrogen of an acid has been replaced by a metal; a rule is that when an acid reacts with an alkali the product is a salt plus water. For example, the formula of hydrochloric acid is HCl—one atom of hydrogen and one of chlorine. The acid reacts with sodium, Na, to make common salt or sodium chloride, NaCl. The hydrogen has been replaced by sodium. In this case the hydrogen is released as a gas. For an example of a reaction between an acid and a base, one may again take hydrochloric acid, this time reacting with caustic soda or sodium hydroxide, NaOH—one atom each of sodium, oxygen, and hydrogen. The reaction is:HCl+NaOH→NaCl+H2O

Again, common salt is formed, leaving two atoms of hydrogen and one of oxygen, which form water, H₂O.

Things are not always that simple. Often only some of the hydrogen of the acid is replaced. The result is an ‘acid salt’ with some available hydrogen which has an acid effect when the salt is dissolved in water. An example is cream of tartar (potassium hydrogen tartrate), whose formula is C₄O₆H₅K. K is the symbol for potassium.

Salts are conventionally named after the metal and the acid that could be used to make them; for example the hard mineral in bone, calcium phosphate, could be made by a reaction between calcium and phosphoric acid. The ending ‘-ide’ denotes a compound of two elements only (such as sodium chloride); ‘-ate’ means that some oxygen is present; ‘-ite’ means that there is a smaller amount of oxygen. Any other extras are added to the name (as in potassium hydrogen tartrate). Making this scheme uniform has involved changing some of the traditional names of salts.

Other salts used in the kitchen include saltpetre (sodium or potassium nitrate), bicarbonate of soda, alum (aluminium potassium sulphate, a ‘double salt’ containing two metals), and waterglass (sodium silicate), formerly used to seal the shells of eggs to make them keep longer. Sea salt is mostly sodium chloride but also contains magnesium chloride, magnesium sulphate (better known as Epsom salts), potassium sulphate, calcium carbonate (dissolved limestone), potassium bromide, and sodium bromide, among many other salts.

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