Science · Chemistry basics
Acids & bases
pH scale, indicators, neutralisation.
What are acids and bases?
Acids are substances that taste sour (like lemon juice) and can sting skin. Bases are substances that feel slippery (like soap) and taste bitter. Scientists never taste or touch chemicals directly – they use tests instead. Strong acids and bases can be dangerous, while weak ones are safe in everyday life.
The pH scale
The pH scale measures how acidic or basic a substance is. It runs from 0 to 14. A pH of 7 is neutral (neither acid nor base), like pure water. Numbers below 7 are acids (lower numbers mean stronger acids). Numbers above 7 are bases or alkalis (higher numbers mean stronger bases).
Indicators
Indicators are special dyes that change colour when mixed with acids or bases. Universal indicator turns red in strong acids, yellow in weak acids, green in neutral, blue in weak alkalis, and purple in strong alkalis. Litmus paper is simpler: it turns red in acid and blue in base.
Neutralisation reactions
When an acid and base react together, they cancel each other out – this is called neutralisation. The reaction makes a salt (not just table salt!) and water. For example, hydrochloric acid plus sodium hydroxide makes sodium chloride and water. Neutralisation is useful for treating stings and stomach aches.
Worked examples
1. Vinegar has a pH of 3. Baking soda solution has a pH of 9. What happens when you mix them?
- Vinegar (pH 3) is an acid because it's below 7.
- Baking soda (pH 9) is a base because it's above 7.
- When an acid and base mix, neutralisation happens.
- They react to make a salt and water, plus bubbles of carbon dioxide gas.
2. A student tests a liquid with universal indicator and it turns yellow. What can she conclude?
- Universal indicator turns yellow in weak acids.
- The pH must be between 4 and 6 (just below neutral).
- Examples of weak acids include fizzy drinks, rain water, and orange juice.
3. Why do gardeners sometimes add lime (a base) to soil?
- Some soils become too acidic for plants to grow well.
- Lime is a base that neutralises the acid in the soil.
- This raises the pH closer to neutral (around pH 7), which most plants prefer.
Try it yourself
A bee sting is acidic. Which household substance would best neutralise it: lemon juice (pH 2), water (pH 7), or baking soda paste (pH 9)?
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