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Acids and bases

Sulphuric acid on a piece of cotton towelSulphuric acid on a piece of cotton towelMany acids and bases are highly reactive chemicals: they easily combine with other substances in chemical reactions to form new ones. Most bases do not dissolve in water; those that do are called alkalis. A weak alkali has a bitter taste, such as the caffeine in coffee. A weak acid usually has a sharp or sour taste, like the citric acid in citrus fruits such as lemons. Strong, or concentrated, acids and alkalis are so reactive that they are corrosive: they dissolve substances—including human skin—to cause severe chemical burns. Examples are the sulphuric acid in a car battery and the alkali sodium hydroxide (caustic soda), which is used as a drain-cleaner.

Mono Lake, California, USA, an alkaline lakeMono Lake, California, USA, an alkaline lake

Donors and receptors

Acids are substances with hydrogen in their molecules. For example, sulphuric acid is H2SO4 and hydrochloric acid is HCl. In solution with water, the hydrogen forms a positive ion: an atom without its electron. An acid is reactive because it is always ready to give up, or donate, this proton in a chemical change in order to become neutral. Alternatively, the acid can accept an electron, which is negative, to achieve the same result. For this reason, acids are known as proton donors or electron receptors. An alkali does the opposite, and so is a proton receptor or electron donor.

Phosphoric acid is used as a rust remover—and also as a flavouring in cola drinks.

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