Swedish scientist Svante Arrhenius won acclaim for his research into how compounds form ions in solution. This work led him to realize that it is hydrogen ions that give acids their special properties.

Lemon juice and vinegar tastesour because they contain weak acids. An acid is a substance that dissolves in water to form positively charged particles called hydrogen ions (H+). The opposite of an acid is an alkali, which dissolves in water to form negatively charged ions of hyfrogen and oxyegen, called hydroxide ions (OH-). Alkalis are “anti-acids” because they cancel out acidity. Toothpaste, for example contains an alkali to cancel out acidity in the mouth that would otherwise damage teeth.

Even the weakest acids cannot be stored in metal containers because acids are corrosive to most metals. When an acid reacts with a metal, hydrogen gas is given off and the metal dissolves in the acid to form a compound called a salt. The reaction is very violent with metals such as potassium and sodium and quite vigorous with metals such as magnesium and zinc.

The more hydrogen ions an acid forms in water, the stronger it is, and the lower its pH. Strong acids, such as sulphuric acid and nitric acid, are very dangerous and must be handled carefully.

Sulphuric acid: concentrated sulphuric acid will dehydrate (remove water from) any substance with which it comes into contact. For example, the acid dehydrates sugar, a carbohydrate, to leave a mass of smouldering black carbon.

Nitric acid: organic matter, such as paper, cork, rubber, fabric and skin is rapidly decomposed by metric acid. The acid is so corrosive because it oxidizes (supplies oxygen to) any material with which it comes into contact.

Salts: when the hydrogen in an acid is replaced by a metal during a chemical reaction, a neutral compound called a salt is formed. For example, when copper reacts with nitric acid, the copper takes the place of the hydrogen to make the salt copper nitrate. Like other metals, copper forms a variety of salts when mixed with different acids. Most salts are crystals and many are colored. Some salts such as sodium chloride (common salt) occur naturally.