Often feel soapy or slippery, and taste bitter — like baking soda solution. (Bitterness alone isn't proof — bitter gourd is bitter but not basic.)
Key idea
Red Rose Extract
Crush red rose petals, soak them in hot water, and filter the coloured liquid — a natural acid-base indicator.
In acid
Turns red.
In base
Turns green. (Beetroot, purple cabbage, turmeric, and jamun extracts also work as indicators.)
Key idea
Turmeric as an Indicator
Turmeric paper (filter paper dipped in turmeric paste and dried) turns red only in basic solutions — it stays unchanged in both acidic and neutral ones.
A familiar exampleA curry (turmeric) stain changes colour when soap — a base — is applied to it.
Key idea
Olfactory Indicators
Some substances change their odour, not colour, in acidic or basic conditions — these are called olfactory indicators.
Why this matters
Onion-soaked cloth changes smell with acids or bases — useful for someone who can't rely on seeing a colour change.
Key idea
Comparing the Indicators
Indicator | In acid | In base | In neutral
Litmus paperBlue → Red — Red → Blue — No change
Red rose extractRed — Green — No change
Turmeric paperNo change — Turns red — No change
Key idea
Neutralisation Reactions
When an acid is mixed with a base in the right amount, the result is neither acidic nor basic.
Acid + Base → Salt + Water + Heat
A neutralisation reaction produces salt and water, and releases heat.
Key idea
Neutralisation in Daily Life
1
An ant bite
Injects acidic formic acid. Rubbing moist baking soda (a base) neutralises it and eases the pain.
→
2
Acidic soil
From excess fertiliser, treated with lime — a base.
→
3
Factory waste
Acidic waste is neutralised with a base before release, to protect fish and other aquatic life.
Chapter · Key terms to remember
Key Terms
Litmus
A natural dye, used as an acid-base indicator.
Acid-base indicator
Shows different colours in acids and bases.
Acidic
Turns blue litmus red; generally sour.
Chapter · Key terms to remember
More Key Terms
Basic
Turns red litmus blue; often soapy, bitter.
Neutral
Changes neither litmus paper.
Olfactory indicator
Changes smell in acid or base.
Chapter · Key terms to remember
More Key Terms
Neutralisation reaction
Acid + Base → Salt + Water + Heat.
Questions for your notebook
Write these down, then discuss
1
A solution turns red litmus paper blue. Excess addition of which solution would reverse the change — lime water, baking soda, vinegar, or common salt solution?
2
Solutions A, B, and C are unknown. Red litmus added to A turns blue; turmeric added to B turns red; red rose extract added to C turns green. What is the nature of A, B, and C?
3
A liquid sample does not change red litmus, turns blue litmus red, and shows no change with turmeric. Is it acidic or basic? Justify.
Project as-is — students copy the questions, then the class discusses answers together.