A scale from a brewery
Chemists measure how acidic or alkaline something is on the pH SCALE, which runs from 0 to 14. In 1909 a Danish chemist, Søren Sørensen, came up with it while working at the laboratory of the Carlsberg brewery in Copenhagen, where knowing exactly how acidic a brew was mattered. Below 7 is ACID, 7 is NEUTRAL, like pure water, and above 7 is a BASE; a base that dissolves in water is called an ALKALI. Each step on the scale is ten times stronger than the last, so an acid of pH 3 is ten times more acidic than one of pH 4.
Acids taste SOUR, like LEMON juice, around pH 2, and VINEGAR. Bases often taste BITTER and feel slippery, like SOAP. Your stomach is strongly acidic, and our birds of prey page covers a bird whose stomach is more acidic still.
Colours that tell the answer
An INDICATOR is a substance that changes colour depending on whether it is in an acid or a base. LITMUS, one of the oldest, is made from lichens, and turns red in acid and blue in alkali. UNIVERSAL indicator shows a whole rainbow of colours along the pH scale. You can make an indicator at home by boiling chopped red cabbage: its purple juice turns pink in vinegar and green or yellow in a SOLUTION of baking soda.
Why baking soda fizzes
Mix an acid with a base and they cancel each other out, making a salt and water. This REACTION is called neutralisation. Indigestion tablets are mild bases that neutralise extra stomach acid. Baking soda, sodium bicarbonate, is a base, and when it meets an acid such as vinegar, lemon juice or buttermilk it also gives off carbon dioxide gas, and that is the fizz. In a cake, those bubbles get trapped in the batter as it heats and set, and the cake rises. Our baking page covers the rest of the kitchen science.
Strong, dilute and dangerous
A STRONG acid is not the same as a concentrated one. Strength is about how completely the acid splits apart in water, releasing HYDROGEN particles; concentration is about how much acid is in the water. Adding water makes a solution more DILUTE. SULFURIC acid, the most produced chemical in the world, is used to make fertilisers and car batteries and is very CORROSIVE, able to burn skin and eat through metal.
Household chemicals need care too. BLEACH and AMMONIA are both used for cleaning, but they must never be mixed, because together they give off a poisonous gas. People have made soap for thousands of years by boiling fats with a strong alkali; clay tablets from ancient Babylon, from around 2800 BC, describe a recipe. Rain is naturally slightly acidic, strong enough to carve out caves in limestone over thousands of years.
The oceans are changing too. Seawater is slightly alkaline, but it absorbs a large share of the carbon dioxide released by burning coal, oil and gas, which forms a weak acid. Since the 1800s the sea's pH has fallen by about 0.1, which on the pH scale means it has become around 30 per cent more acidic. That makes it harder for corals, shellfish and tiny plankton to build their shells and skeletons.