Acids, Bases and Salts - Class 7 Science (CBSE)
Based on the 2025-26 NCERT syllabus for Class 7 Science. This chapter introduces acidic and basic substances, natural indicators to identify them, and the process of neutralisation.
1. Why this chapter matters
Acids and bases are everywhere -- in the food we eat, cleaning products we use, and even in our stomachs. Understanding them helps us handle substances safely. In CBSE exams, this chapter contributes 6-8 marks with a focus on indicators and neutralisation.
2. Acids
Acids are substances that have a sour taste. They turn blue litmus paper red.
Natural sources of acids
| Acid | Found in |
|---|---|
| Citric acid | Lemon, orange, grapefruit |
| Acetic acid | Vinegar |
| Tartaric acid | Tamarind, grapes |
| Lactic acid | Curd, sour milk |
| Oxalic acid | Spinach, tomato |
| Ascorbic acid (Vitamin C) | Amla, citrus fruits |
Properties of acids
- Sour taste.
- Corrosive (can damage skin and materials).
- Turn blue litmus red.
- React with bases to form salt and water.
3. Bases
Bases are substances that have a bitter taste and feel soapy to touch. They turn red litmus paper blue.
Examples of bases
| Base | Found in/used as |
|---|---|
| Sodium hydroxide (NaOH) | Soap, drain cleaner |
| Calcium hydroxide (lime water) | Whitewash |
| Magnesium hydroxide | Antacid (milk of magnesia) |
| Ammonium hydroxide | Window cleaner |
| Potassium hydroxide | Soap making |
Properties of bases
- Bitter taste.
- Soapy or slippery feel.
- Turn red litmus blue.
- React with acids to form salt and water.
4. Natural indicators
An indicator is a substance that shows whether a given substance is acidic or basic by changing colour.
Litmus
Litmus is obtained from lichens (small plants). It is available as litmus paper (red and blue) or litmus solution.
| Substance | Blue litmus | Red litmus |
|---|---|---|
| Acid | Turns red | Stays red |
| Base | Stays blue | Turns blue |
| Neutral | Stays blue | Stays red |
Turmeric
Turmeric is a yellow powder used as a spice. It acts as a natural indicator.
- With acids: Remains yellow.
- With bases: Turns reddish-brown.
China rose (Hibiscus)
China rose petals can be used to prepare an indicator solution.
- In acids: Turns dark pink or magenta.
- In bases: Turns green.
Phenolphthalein
Phenolphthalein is a synthetic indicator.
- In acids: Colourless.
- In bases: Pink.
5. Indicator colour change summary
| Indicator | In acid | In base |
|---|---|---|
| Blue litmus | Red | No change (stays blue) |
| Red litmus | No change (stays red) | Blue |
| Turmeric | Yellow | Reddish-brown |
| China rose | Dark pink/magenta | Green |
| Phenolphthalein | Colourless | Pink |
6. Neutralisation
Neutralisation is the reaction between an acid and a base to form salt and water.
Acid + Base = Salt + Water
Heat is also released during neutralisation (exothermic reaction).
Example
Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) = Sodium chloride (NaCl) + Water (H2O)
7. Applications of neutralisation
Antacids for indigestion
When the stomach produces too much acid, it causes indigestion. Antacids (containing magnesium hydroxide or aluminium hydroxide) neutralise the excess acid.
Soil treatment
If soil is too acidic, farmers add lime (calcium hydroxide) or quicklime (calcium oxide) to neutralise it. If soil is too basic, they add organic matter (compost) which is slightly acidic.
Bee sting
Bee sting contains formic acid (an acid). It is treated by applying baking soda (sodium hydrogen carbonate), which is a base.
Wasp sting
Wasp sting is basic. It is treated with dilute vinegar (acetic acid), which is an acid.
Factory waste
Factory waste is often acidic. It must be neutralised with a base before being released into water bodies to prevent harm to aquatic life.
8. Worked examples
Example 1: A substance turns blue litmus red. Is it an acid or a base?
It is an acid. Blue litmus turns red only in acids.
Example 2: Two solutions X and Y are tested with turmeric. X turns turmeric reddish-brown, Y keeps it yellow. Identify X and Y.
X is a base (turmeric turns reddish-brown with bases). Y is either an acid or neutral (turmeric stays yellow).
Example 3: Why is baking soda applied on a bee sting area?
Bee sting injects formic acid. Baking soda (a mild base) neutralises the acid, providing relief.
9. Common mistakes and how to fix them
| Mistake | Fix |
|---|---|
| Thinking all acids are dangerous | Many natural acids (citric, acetic) are safe in small amounts |
| Confusing acid-base indicator colour changes | Memorise each indicator's colour in acid and base separately |
| Believing neutralisation always produces neutral salts | Some salts can be acidic or basic (e.g., sodium carbonate is basic) |
| Using the wrong substance for a wasp sting | Wasp sting is basic, so apply acid (vinegar), not base |
| Thinking phenolphthalein works in acids | Phenolphthalein is colourless in acids; turns pink in bases only |
10. CBSE exam focus
| Question type | Marks | Frequency |
|---|---|---|
| Identify acid/base using indicators | 2 marks | 1 question |
| Indicator colour change table | 2-3 marks | 1 question |
| Neutralisation definition and equation | 2 marks | 1 question |
| Applications of neutralisation | 3 marks | 1 question |
| Natural sources of acids | 1-2 marks | 1 question |
11. Self-test
- What colour does red litmus turn in a base?
- Name the natural indicator obtained from lichens.
- What is neutralisation? Write a general equation.
- Why is lime added to acidic soil?
- A substance turns china rose indicator green. Is it an acid or a base?
- Differentiate between a bee sting and a wasp sting treatment.
12. Answer key
- Red litmus turns blue in a base.
- Litmus.
- Acid + Base = Salt + Water. Example: HCl + NaOH = NaCl + H2O.
- Lime is basic. It neutralises the excess acid in the soil, making it suitable for plant growth.
- It is a base. China rose turns green in bases and dark pink/magenta in acids.
- Bee sting: acidic (formic acid) -- treat with baking soda (base). Wasp sting: basic -- treat with vinegar (acid).
13. Quick revision
- Acids: sour, turn blue litmus red.
- Bases: bitter, soapy, turn red litmus blue.
- Indicators: litmus, turmeric, china rose, phenolphthalein.
- Neutralisation: Acid + Base = Salt + Water.
- Applications: antacids, soil treatment, insect stings, factory waste.
- Natural indicators come from plants (lichens, turmeric, hibiscus).
- Bee stings are acidic; wasp stings are basic.
