ACIDS AND BASES
Some substances and chemicals are very reactive. This means they easily combine or join with other substances in chemical reactions, to form new substances. Two types of reactive chemicals are acids and bases. Many bases dissolve in water: these are called alkalis. Strong or concentrated acids and alkalis are so reactive that they are corrosive. This means they break down and dissolve substances, including human skin, to cause severe chemical burns. Examples are the sulphuric acid in a car battery and the alkali sodium hydroxide, which is used as a drain-cleaner. Strong alkalis may also be slippery or slimy, almost like thin jelly.
A weak acid usually has a sharp or sour taste, like the natural citric acid in citrus fruits such as lemons and grapefruits. A weak alkali has a bitter taste, such as the caffeine in coffee. Caffeine is an example of an alkaloid – a natural alkali found in certain plants. Many plants make alkaloids in their leaves and stems. These are poisonous so that animals avoid eating them. Some animals have bites or stings that inject poisonous acid into prey or enemies.
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, H+. This hydrogen ion is, in fact, a hydrogen atom without its electron – that is, it is just a proton. An acid is reactive because it is always ready to give up, or donate, this proton in a chemical change, in order to rid itself of the positive charge and become neutral. Alternatively, the acid can accept an electron, which is negative, to achieve the same result. This is why acids are known as proton donors or electron receptors. An alkali does the opposite and so is a proton receptor or electron donor.
The fumes from vehicles exhausts, factories and power stations contain nitrogen and sulphur chemicals. These dissolve in the drops of water in clouds to form nitric and sulphuric acids which fall as acid rain.
The human body contains a strong acid. This is hydrochloric acid, found in the stomach. It is made in the stomach lining in an inactive form, and released when food enters the stomach during a meal. The acid is very corrosive and helps to break down the nutrients in food as part of digestion. It also helps to kill any dangerous microbes (germs) that might be in the food. The stomach lining protect itself from being dissolved by its own acid by making a thick layer of slimy mucus.
A bee’s sting contains a mix of poisonous substances including acidic apitoxin. The bee leaves its barb and poison sack in the skin. The sting can be treated by rubbing on a weak solution of bicarbonate of soda, an alkali that counteracts the acid.
Like a bee, a wasp has a painful sting – also acidic, rather than alkaline as is popularly believed. The pain can be treated with a weak solution of ammonia, which is an alkali that neutralizes the acid in the sting.
Acids and alkalis are widely used in industry. Millions of tonnes of sulphuric acid are produced every year, not only for vehicle batteries but for processes such as making detergents, explosives, fertilizers and dyes for colouring. Sulphuric acid is also used in the splitting or refining of crude oil (petroleum) to make petrol, diesel, plastics, paints and other petroleum products.
ACID OR ALKALI?
Touching or tasting an unknown liquid to find out if it is an acid or alkali is far too dangerous – even deadly. The usual way is to use an indicator. This is a substance which changes its colour when added to an acid or alkali. One of the best-known indicators is litmus. It can be used as a liquid or on a dry paper strip. Normally litmus is a pinky colour. When it is added to an acid it turns red. Added to an alkali it becomes blue. If litmus does not change colour when added to a substance then the substance is neither acid nor alkali but neutral. Normal rainwater is a very weak natural acid because it contains tiny amounts of dissolved carbon dioxide, one of the gases in air, which forms carbonic acid. Acid rain from pollution is much stronger.
Acids and alkalis are very important in farming and forestry. Some soils are naturally slightly acidic, others are slightly alkaline. Each type of plant grows best in a certain type of soil, acidic or neutral or alkaline. To grow a certain plant the soil can be changed using additives. Adding alkaline lime to an acidic soil makes it more neutral.
In medieval times the terrible disease plague killed millions. The dead bodies were sprinkled with lime, a strong and corrosive base. It helped to kill the germs in the bodies and make them rot faster.
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