Acids and Bases

Bite into a lemon and your face scrunches up. Rub soapy water between your fingers and it feels slippery. Those two sensations come from opposite ends of one of the most useful ideas in chemistry: the difference between acids and bases.

What makes an acid

An acid is a substance that releases hydrogen ions when it dissolves in water. A hydrogen ion is simply a hydrogen atom that has lost its electron, leaving a single bare proton. These protons are very reactive. They give acids their sour taste and their ability to eat into metals, rocks and skin.

Many acids are weak and harmless. Citric acid gives lemons and oranges their tang, acetic acid makes vinegar sharp, and carbonic acid gives fizzy drinks a bite. Others are strong and dangerous. Hydrochloric acid, which your stomach makes to break down food and kill germs, would burn your skin. Sulphuric acid in car batteries is more corrosive still.

What makes a base

A base is the chemical opposite: it removes hydrogen ions, often by releasing hydroxide ions, which grab protons and turn them into water. Bases that dissolve in water are called alkalis. They taste bitter and feel slippery because they react with the oils on your skin, turning them into a kind of soap. Baking soda, soap, toothpaste and antacid tablets are mild bases. Bleach, oven cleaner and drain cleaner are strong ones and need to be handled with care.

The pH scale

Chemists measure how acidic or basic something is on the pH scale, which runs from 0 to 14. Pure water sits at 7, which is neutral. Anything below 7 is acidic, and anything above is basic. The scale is logarithmic, meaning each step is ten times stronger than the last. Lemon juice at pH 2 is not just a bit more acidic than tomato juice at pH 4; it is a hundred times more acidic.

You can measure pH with an electronic meter or with an indicator, a dye that changes colour. Litmus paper turns red in acid and blue in alkali. Universal indicator shows a whole rainbow. Even red cabbage juice works: it turns pink in vinegar and green in soapy water.

Neutralisation

When an acid meets a base, they cancel each other out. The hydrogen ions from the acid and the hydroxide ions from the base join to make water, and the leftover pieces form a salt. This is called neutralisation. An antacid tablet eases heartburn by neutralising extra stomach acid. Farmers spread lime, which is basic, on fields whose soil has become too acidic for crops. A bee sting is slightly acidic, which is why some people dab it with a paste of baking soda.

Acids and bases in your body

Your body controls pH with great precision. Stomach acid is fiercely acidic, but the intestine that follows is slightly alkaline, because the pancreas pours in bicarbonate to neutralise the acid. Your blood must stay between pH 7.35 and 7.45. Chemicals called buffers soak up extra acid or base, and your lungs and kidneys adjust how much carbon dioxide and acid you get rid of. If blood pH strays far outside its narrow range, cells stop working properly.

Why it matters

pH shapes the natural world. Acid rain, caused by pollution, has damaged forests and lakes. Today the oceans are absorbing carbon dioxide from the air, which forms carbonic acid and slowly lowers their pH. More acidic water makes it harder for corals, shellfish and plankton to build their shells. Understanding acids and bases helps us cook, clean, farm, keep healthy and protect the seas.