Materials (Classes VI–VIII) • Topic 4 of 5

Acids, Bases & Salts

From NCERT Class 7 'Acids, Bases and Salts'. ACIDS taste SOUR and turn BLUE litmus RED. Everyday acids: citric acid (lemon, orange), acetic acid (vinegar), tartaric acid (tamarind, imli), and in the stomach hydrochloric acid; ant and nettle stings contain formic acid. BASES (alkalis when soluble) taste BITTER and feel SOAPY and turn RED litmus BLUE. Examples: soap, baking soda (sodium bicarbonate), washing soda, lime water (calcium hydroxide), and milk of magnesia. INDICATORS are substances that show whether something is acidic or basic by a colour change: LITMUS (natural, from lichen — blue litmus → red in acid, red litmus → blue in base), turmeric (yellow → red in a base), and china rose. A NEUTRAL substance (neither acid nor base) does NOT change the colour of litmus — pure water and a common salt (sodium chloride) solution are neutral. NEUTRALISATION is the reaction between an acid and a base giving a SALT and WATER, with heat released; the acidic and basic natures cancel out. Everyday neutralisation CTET loves: an antacid (a mild base) neutralises excess stomach acid (acidity/indigestion); applying baking soda (base) or moist soap relieves an acidic ant/bee sting; lime/calamine soothes a basic nettle/wasp sting; and farmers add lime (a base) to neutralise acidic soil. MEMORY ANCHOR: 'Acid → blue to red; Base → red to blue.' PEDAGOGY/MISCONCEPTION: pupils mix up which way litmus turns, and assume 'salt' means only table salt — in chemistry a salt is the product of any acid–base reaction. The teacher's natural-indicator activity (red cabbage / china rose / turmeric) makes the idea concrete and memorable. HOW TESTED: 'what colour does X litmus turn', identify the acid/base in a food, or name the everyday neutralisation.

✅ Solved examples

1. A student dips BLUE litmus paper into lemon juice. What colour change occurs and what does it indicate?
The blue litmus turns RED, indicating that lemon juice is ACIDIC (it contains citric acid). Acids turn blue litmus red.
2. Soap solution turns RED litmus paper blue. Soap is therefore:
A base (basic in nature). Bases turn red litmus blue and feel soapy/slippery.
3. A person with acidity (excess acid in the stomach) takes an antacid for relief. The chemistry behind this is:
Neutralisation — the antacid is a mild base that neutralises the excess stomach acid, forming a salt and water and cancelling the acidic effect.
4. Common salt (sodium chloride) solution is tested with both red and blue litmus and neither colour changes. The solution is:
Neutral — it is neither acidic nor basic, so it does not change the colour of either litmus. It is the salt formed from an acid–base (HCl + NaOH) neutralisation.

✏️ Practice — try these, take hints as needed

1. What colour does RED litmus turn when dipped in lime water (a base)?
Bases reverse red litmus.
Red to ____.
Blue
2. Tamarind (imli) tastes sour and reddens blue litmus. The acid present in it is:
Named after tartar.
Also found in grapes.
Tartaric acid
3. An ant sting injects formic acid and causes burning. Which household substance is rubbed on for relief, and by what process?
A mild base such as baking soda or moist soap.
It cancels the acid.
Baking soda (a base) — it neutralises the acidic sting (neutralisation)
4. Turmeric and china rose are examples of substances used to detect acids and bases. Such substances are collectively called:
They show a colour change.
Litmus is the most common one.
Indicators

📝 Topic test — 8 questions

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