Acids and Bases

A tale of two opposites

Bite into a lemon and your face scrunches up at the sharp, sour taste. Touch a bar of soap and it feels strangely slippery. These two everyday experiences are your senses meeting two of the most important groups of substances in chemistry: acids and bases. They are opposites in many ways, and the dance between them shapes everything from the food you digest to the soil that grows your vegetables.

Understanding acids and bases starts with knowing they are chemical opposites. An acid is one kind of substance; a base is the other. And just as cold and hot can be measured on a temperature scale, the "acidness" or "baseness" of a substance can be measured on a special scale of its own.

The pH scale

Chemists measure how acidic or basic a substance is using the pH scale, which runs from 0 to 14.

• A pH below 7 means the substance is an acid. The lower the number, the stronger the acid.
• A pH of exactly 7 means the substance is neutral — neither acid nor base. Pure water sits here.
• A pH above 7 means the substance is a base (a base that dissolves in water is called an alkali). The higher the number, the stronger the base.

Here is roughly where some everyday substances fall:

Substance	pH (approx.)	Type
Battery acid	0–1	Strong acid
Stomach acid	1–2	Strong acid
Lemon juice	2	Acid
Vinegar	3	Acid
Pure water	7	Neutral
Baking soda solution	9	Base
Soap	9–10	Base
Oven cleaner	13–14	Strong base

One important detail: each step on the pH scale represents a tenfold change in strength. A substance at pH 3 is ten times more acidic than one at pH 4, and a hundred times more acidic than pH 5. Small numbers, big differences.

What makes an acid an acid?

So what is actually happening inside these substances? When an acid dissolves in water, it releases tiny charged particles called hydrogen ions (written H⁺). The more hydrogen ions an acid releases, the stronger and more acidic it is. These ions are the source of an acid's sharp, sour taste and its ability to react with metals and other materials. (Acids and bases are built from atoms joining and splitting apart — a foundation you can explore in Atoms and Molecules.)

Bases work in the opposite way. When a base dissolves in water it releases or attracts particles that soak up hydrogen ions — usually releasing hydroxide ions (written OH⁻). Bases feel slippery because they react with the natural oils on your skin, and strong ones can be just as harmful as strong acids.

Indicators: revealing the hidden

You cannot tell an acid from a base just by looking, and you should certainly never taste or touch an unknown chemical to find out. Instead, chemists use indicators — substances that change colour depending on whether they meet an acid or a base.

The most famous is litmus: litmus paper turns red in acid and blue in base. A handy memory trick is "acid turns litmus red." A more colourful indicator is universal indicator, which shows a whole rainbow of colours for different pH values, from red (strong acid) through green (neutral) to purple (strong base).

Remarkably, some indicators grow in nature. The purple-blue juice of red cabbage changes colour with pH — turning pink in acid and green or yellow in base — which makes it a perfect indicator for a safe home experiment.

Neutralisation: when opposites cancel out

Here is where the story gets useful. When you mix an acid and a base together, they react and cancel each other out. This reaction is called neutralisation, and the general result is always the same:

acid + base → water + a salt

The hydrogen ions from the acid combine with the hydroxide ions from the base to make plain, neutral water, while the leftover parts join to form a salt. (Table salt, sodium chloride, is just one of many possible salts.) The fierce acid and the harsh base destroy each other's properties, leaving something gentle behind.

Neutralisation is everywhere in real life:

• Antacid tablets contain a mild base. When you have an upset stomach from too much acid, an antacid neutralises the excess and eases the burning.
• Toothpaste is mildly basic. The bacteria in your mouth produce acids that attack tooth enamel, and toothpaste helps neutralise them.
• Farmers add powdered lime (a base) to soil that has become too acidic, so crops can grow well.
• Bee and wasp stings differ: a bee sting is acidic, so a mild base can soothe it, while a wasp sting is more basic.

Why acids and bases matter

The balance between acids and bases is not just a laboratory curiosity — it is a matter of life and death. Your blood must stay very close to pH 7.4; even a small shift can be dangerous, so your body works constantly to keep it balanced. In nature, when pollution makes rain too acidic, acid rain can damage forests, lakes and stone buildings, which connects to the wider problems explored in Pollution and How to Stop It. And the ocean is slowly becoming more acidic as it absorbs extra carbon dioxide from the air, threatening coral reefs and shellfish.

Try this — the red cabbage indicator

With an adult's help, chop a few leaves of red cabbage and pour hot water over them (an adult should handle the hot water). Let it sit until the water turns deep purple, then strain off the purple liquid — this is your indicator. Pour a little into several clear cups. Now add a different household substance to each: lemon juice or vinegar, water, a pinch of baking soda dissolved in water, and a little soapy water. Watch the colours change! Acids turn the cabbage juice pink or red, neutral leaves it purple, and bases turn it blue, green or yellow. You have built a real pH indicator from a vegetable. Wear old clothes (it stains), keep everything away from your eyes and mouth, and wash your hands when you finish.

From the sour zing of a lemon to the medicine that soothes a sore stomach, acids and bases are a hidden chemistry working all around you — and now you can measure, test and understand it.