Acids, Bases and Chemical Changes: Class 7 Study Guide
Acids taste sour and bases feel soapy. Indicators change colour to tell them apart.
Acids taste sour. Lemon and orange contain citric acid, vinegar contains acetic acid and curd contains lactic acid. Bases taste bitter and feel soapy, as in soap, baking soda and lime water. Never taste unknown chemicals, because many are harmful.
An indicator is a substance that changes colour in acids and bases. Litmus turns red in acid and blue in base. Turmeric stays yellow in acid and turns red in base. China rose petals turn magenta in acid and green in base.
Neutralisation
When an acid and a base mix, they cancel each other's effect. This is neutralisation, and it forms a salt and water. Examples: an antacid (milk of magnesia) soothes acidity in the stomach; baking soda on an ant sting soothes the formic acid; a farmer adds lime to acidic soil.
| Situation | Acid | Neutralised by |
|---|---|---|
| Indigestion | Excess stomach acid | Antacid tablet |
| Ant sting | Formic acid | Baking soda or calamine |
| Acidic soil | Acid in soil | Quicklime or slaked lime |
Physical and chemical changes
In a physical change, no new substance forms, as in melting ice or dissolving sugar. In a chemical change, a new substance forms, as when iron rusts, a candle burns, milk sours or food cooks. Rusting needs both oxygen and water, so paint, oil and galvanising protect iron.
Gas, colour, smell, heat?
Usually physical changes can.
Physical or chemical.
Say what was formed.
Crystallisation
A crystal forms when a hot, saturated solution cools. Copper sulphate crystals grown from a solution are bright blue. This method is also used to purify salt from sea water.
Worked examples and common mistakes
Example: Red litmus turns blue in a base. Example: Is burning a candle a chemical change? Yes, new substances form. Common mistakes: thinking that all acids are dangerous (many are in our food), and calling dissolving a chemical change. It is physical.
Check your understanding: model answers
1. Red. 2. It neutralises the formic acid. 3. Physical.
✨ Key Takeaways
- Litmus turns red in acid and blue in base.
- Neutralisation gives a salt and water.
- Rusting and burning are chemical changes.
- Never taste or touch unknown chemicals.
💬 Questions for Discussion & Reflection
- What colour does blue litmus turn in acid?
- Why is baking soda used on an ant sting?
- Is melting wax a physical or chemical change?
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