Science
What are acids and bases?
Quick answer
Acids are sour substances with a pH below 7, like lemon juice. Bases are slippery or bitter with a pH above 7, like soap. When an acid and a base react they neutralise each other.
In one sentence
Acids and bases are two opposite kinds of chemicals that can be measured on the pH scale, and they cancel each other out when mixed.
Explained
Many everyday liquids are acids. Lemon juice, vinegar and fizzy drinks taste sour. Your stomach makes strong hydrochloric acid to break down food and kill germs. Bases are the chemical opposites. Soap, toothpaste and baking soda are bases. A base that dissolves in water is called an alkali. Bases often feel slippery and taste bitter, but never taste or touch chemicals to find out, because strong acids and bases are dangerous and can burn skin.
The pH scale runs from 0 to 14 and tells you how acidic or alkaline something is.
- Below 7: acid. The lower the number, the stronger the acid.
- 7: neutral, such as pure water.
- Above 7: alkaline. The higher the number, the stronger the alkali.
lemon
soap
magnifying glass
| Substance | pH (about) |
|---|---|
| Stomach acid | 1 to 2 |
| Lemon juice | 2 |
| Vinegar | 3 |
| Pure water | 7 |
| Baking soda solution | 9 |
| Soap | 9 to 10 |
| Bleach | 12 to 13 |
We test with indicators, which change colour. Litmus paper turns red in acid and blue in alkali. Universal indicator gives a range: red or orange for acids, green for neutral, blue or purple for alkalis. Red cabbage juice works as a home-made indicator: it turns pink with acid and blue-green with alkali.
When an acid and a base are mixed in the right amounts, they react and cancel out. This is neutralisation. The pH moves toward 7, and the products are a salt and water. Indigestion tablets use a base to neutralise extra acid in the stomach, and farmers add lime to acid soil.
Acids and bases are examples of chemical reactions. The particles inside are atoms, and some of what we eat and use daily comes from materials that are acids or bases.
Examples
| Everyday item | Acid, base or neutral? |
|---|---|
| Lemon juice | acid |
| Vinegar | acid |
| Water | neutral |
| Baking soda | base |
| Soap | base |
Common mistakes
- "All acids are dangerous." Weak acids, like vinegar and lemon juice, are safe to eat.
- "Alkali and base are always the same." An alkali is a base that dissolves in water.
- Tasting to test. Never do this with chemicals.
Quick quiz
Acids and bases
Check what you know with 4 quick questions.
Question 1 of 4
What is the pH of a neutral substance like pure water?
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