Science · Class 7 · Chapter 2

Exploring Substances: Acidic, Basic, and Neutral

Aim: Build a clear, working understanding of this chapter's key ideas, connected to real NCERT examples, worked problems, and everyday situations.
  • 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.
  • Basic — Turns red litmus blue; often soapy, bitter.
  • Neutral — Changes neither litmus paper.
  • Olfactory indicator — Changes smell in acid or base.
  • Neutralisation reaction — Acid + Base → Salt + Water + Heat.
40 min lesson

Chapter 2 · Concept 1 of 8

Litmus as an Indicator

  • Litmus, obtained from lichens, comes as a solution or as blue and red paper strips.
  • Acidic — Turns blue litmus paper red.
  • Basic — Turns red litmus paper blue.
  • Neutral — Changes neither litmus paper — e.g. tap water, sugar solution.

Chapter 2 · Concept 2 of 8

Taste and Touch as Clues

  • Acidic substances — Generally taste sour — lemon juice, amla juice, tamarind water, vinegar.
  • Basic substances — Often feel soapy or slippery, and taste bitter — like baking soda solution. (Bitterness alone isn't proof — bitter gourd is bitter but not basic.)

Chapter 2 · Concept 3 of 8

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.)

Chapter 2 · Concept 4 of 8

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 example — A curry (turmeric) stain changes colour when soap — a base — is applied to it.

Chapter 2 · Concept 5 of 8

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.

Chapter 2 · Concept 6 of 8

Comparing the Indicators

  • Indicator | In acid | In base | In neutral
  • Litmus paper — Blue → Red — Red → Blue — No change
  • Red rose extract — Red — Green — No change
  • Turmeric paper — No change — Turns red — No change

Chapter 2 · Concept 7 of 8

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.

Chapter 2 · Concept 8 of 8

Neutralisation in Daily Life

  • An ant bite — Injects acidic formic acid. Rubbing moist baking soda (a base) neutralises it and eases the pain.
  • Acidic soil — From excess fertiliser, treated with lime — a base.
  • Factory waste — Acidic waste is neutralised with a base before release, to protect fish and other aquatic life.

Quick Recap

Check Your Understanding

  1. 1A 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. 2Solutions 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. 3A liquid sample does not change red litmus, turns blue litmus red, and shows no change with turmeric. Is it acidic or basic? Justify.