Chapter 2 — Exploring Substances
Acidic, Basic, and Neutral · Class 7 Science
1. What is this chapter about?
Many things around us — fruits, soaps, soil, water — are either acidic, basic (alkaline), or neutral. This chapter teaches us how to identify them safely (without tasting!) using special substances called indicators, and where this knowledge helps in real life — like treating ant bites, balancing soil, or protecting fish from factory waste.
2. Three Types of Substances
🍋 Acidic
- Taste sour
- Contain acids
- Turn blue litmus → red
Examples: lemon, vinegar, curd, tamarind, unripe mango, amla, orange.
🧼 Basic
- Taste bitter
- Feel soapy/slippery
- Turn red litmus → blue
Examples: soap, baking soda, lime water, washing powder.
⚪ Neutral
- Neither acidic nor basic — no change in litmus colour.
- Examples: pure water, sugar solution, common salt solution.
⚠️ Never taste or touch unknown substances — some acids and bases are very harmful. Always use indicators instead.
3. What is an Indicator?
An indicator is a substance that changes its colour (or smell) when it touches an acid or a base. It helps us identify the nature of a substance.
Two types:
- Natural indicators — from plants/lichens: litmus, turmeric, red rose, china rose (gudhal), red cabbage, beetroot, jamun.
- Synthetic indicators — made in labs, e.g. phenolphthalein, methyl orange.
4. Common Indicators & Their Colour Changes
| Indicator | In Acid | In Base | In Neutral |
|---|
| Blue litmus | turns red | no change | no change |
| Red litmus | no change | turns blue | no change |
| Turmeric (yellow) | no change | turns red | no change |
| Red rose extract | red / pink | green | pink |
| China rose (gudhal) | dark pink | green | — |
| Phenolphthalein | colourless | pink | colourless |
Memory trick: "Blue litmus → Base keeps it Blue" · "Red litmus → Red in acid stays Red".
Litmus is obtained from lichens (a plant-like growth on tree bark, formed by fungus + algae living together).
5. Olfactory Indicators
Some substances change their smell in acids or bases — these are called olfactory indicators ("olfactory" = related to smell).
Examples: onion, vanilla extract, clove oil. Useful for visually-impaired students who cannot see colour changes.
6. Acids in Everyday Life
| Acid | Found in |
|---|
| Citric acid | Lemon, orange, amla |
| Acetic acid | Vinegar |
| Lactic acid | Curd, yogurt |
| Tartaric acid | Tamarind (imli), grapes |
| Formic acid | Ant sting, nettle leaves |
| Ascorbic acid (Vit C) | Amla, citrus fruits |
7. Bases in Everyday Life
- Baking soda (sodium bicarbonate) — used in cooking, also neutralises ant stings
- Soap, detergent, shampoo — clean dirt
- Lime water (calcium hydroxide) — used in whitewashing
- Antacids (e.g. milk of magnesia) — neutralise stomach acid
- Toothpaste — neutralises acids made by bacteria in the mouth
Important: Not every bitter thing is a base! Coffee and bitter gourd taste bitter but are not bases.
8. Neutralisation Reaction ⭐ (Important!)
When an acid reacts with a base, they cancel out each other's effect, producing salt and water. Heat is also released. This is called neutralisation.
Acid + Base → Salt + Water + Heat ⚡
The resulting solution is usually neutral (the salt formed may be acidic, basic or neutral depending on which acid and base reacted).
9. Neutralisation in Real Life 🌍
🐜 Ant Sting
Ants inject formic acid into the skin → causes burning. Rubbing moist baking soda (a base) or calamine lotion neutralises it.
🤢 Indigestion / Acidity
Stomach makes too much HCl (hydrochloric acid). We take an antacid (like milk of magnesia) which is a base — it neutralises the extra acid.
🌱 Soil Treatment
- If soil is too acidic → add lime / slaked lime (base).
- If soil is too basic → add organic manure / compost (releases acids).
🏭 Factory Waste
Many factory wastes are acidic. If released directly, they kill fish and pollute rivers. Factories add lime (a base) to neutralise the waste before discharging it.
🦷 Tooth Decay
Bacteria in the mouth produce acid that damages teeth. Toothpaste is basic — it neutralises this acid and protects teeth.
10. Fun Facts 🌟
- Hydrangea flowers change colour with soil — blue in acidic soil, pink in basic soil! Nature's own indicator.
- Lichens grow only in clean air — they're a sign of low pollution.
- Our stomach uses acid (HCl) to digest food and kill germs.
- You can write a secret message using turmeric and reveal it with soap solution!
📌 Chapter Summary in 30 seconds:
Substances are acidic (sour), basic (bitter, soapy), or neutral. We identify them using indicators — natural (litmus, turmeric, rose) or synthetic (phenolphthalein). Some indicators change smell instead of colour (olfactory). When acid meets base, neutralisation happens: Acid + Base → Salt + Water + Heat. This is used in antacids, ant-bite relief, soil correction, factory waste treatment, and toothpaste.