SALT

Abbreviation for Strategic Arms Limitation Talks, a series of US–Soviet negotiations 1969–79.

salt

In chemistry, any compound formed from an acid and a base through the replacement of all or part of the hydrogen in the acid by a metal or electropositive radical. Common salt is sodium chloride.

A salt may be produced by a chemical reaction between an acid and a base, or by the displacement of hydrogen from an acid by a metal (see displacement reaction). As a solid, the ions normally adopt a regular arrangement to form crystals. Some salts form only stable crystals as hydrates (when combined with water). Most inorganic salts readily dissolve in water to give an electrolyte (a solution that conducts electricity).

Formula As all salts are electrically neutral, the formula of a salt can be worked out by making sure that the total numbers of positive and negative charges arising from the ions are equal.

Preparation Various methods can be used to prepare salts in the laboratory; the choice is dictated by the starting materials available and by whether the required salt is soluble or insoluble.

Methods include:

(I) acid + metal for salts of magnesium, iron, and zinc;

(ii) acid + base for salts of magnesium, iron, zinc, and calcium;

(iii) acid + carbonate for salts of all metals;

(iv) acid + alkali for salts of sodium, potassium, and ammonium;

(v) direct combination for sulphides and chlorides;

(vi) double decomposition for insoluble salts.

In methods (i)–(iii) an excess of the solid reactant is added to the acid to ensure that no acid remains. The excess solid is filtered from the salt solution and the filtrate is boiled to a much smaller volume; it is then allowed to cool and crystallize. The salt crystals are filtered and dried on filter paper.

In method (iv) an indicator is used to determine the volume of acid needed to neutralize the alkali (or vice versa). The colour can then be removed by charcoal treatment, or alternatively the experiment can be repeated without the indicator. The solution is boiled to a smaller volume, cooled to crystallize the salt, and the crystals filtered and dried as in (i)–(iii) above.

In method (v) the salt is made in one step and does not require drying.

In method (vi) the two solutions are mixed and stirred. The precipitated salt is filtered, washed well with water to remove the soluble impurities, and allowed to dry in air or an oven at 60–80°C/140–176°F.