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Lemon juice, soap, baking soda, vinegar: Class 7 science asks you to sort everyday substances by how they behave with an indicator. Students often memorise "acid turns litmus red" and then get the questions wrong, because there are two colours of litmus and three indicators in the chapter. This guide sets out what the chapter actually teaches, a reliable way to read any indicator result, worked examples and a short practice plan.
What does Chapter 2 of Class 7 Curiosity cover?
The chapter is Exploring Substances: Acidic, Basic, and Neutral, Chapter 2 of NCERT's *Curiosity* science book for Grade 7 (first edition April 2025; the NCERT copy is marked "Reprint 2026-27"). It covers:
Litmus, a natural dye from lichens, sold as blue and red paper strips.
Red rose extract, made by soaking crushed fallen petals in hot water.
Turmeric paper, made by dipping filter paper in turmeric paste and drying it.
Olfactory indicators, such as onion-soaked cloth strips whose smell changes.
Neutralisation: acid + base gives salt + water + heat.
Everyday uses: baking soda on an ant bite, lime for acidic soil, organic matter for basic soil, and treating acidic factory waste before it reaches a lake.
How do you read an indicator result?
Each indicator has its own rule. Learn them as a short table in a list:
Blue litmus: turns red in an acid. No change in a base or a neutral substance.
Red litmus: turns blue in a base. No change in an acid or a neutral substance.
Red rose extract: a shade of red in an acid, a shade of green in a base.
Turmeric: turns red in a base. Stays yellow in an acid and in a neutral substance.
The key point is the last line. Turmeric can tell you something is basic, but it cannot tell an acid from a neutral substance. The chapter says so directly, and examiners like to test it.
Worked examples
Example 1: three unlabelled bottles. You are given lemon juice, soap solution and salt water in bottles P, Q and R, but not which is which. You have one strip of each litmus.
Put a drop of each on blue litmus. Only P turns it red. So P is acidic: the lemon juice.
Put a drop of Q and R on red litmus. Q turns it blue, so Q is basic: the soap solution.
R changed neither paper. R is neutral: the salt water.
Example 2: reading turmeric. A liquid leaves turmeric paper yellow. Is it acidic? You cannot say. It could be acidic or neutral. Test it on blue litmus: if it turns red, it is acidic; if not, it is neutral.
Example 3: neutralisation by drops. You put one drop of lemon juice in about twenty drops of water and add blue litmus solution. It goes red. You add lime water one drop at a time and swirl. Suppose it turns blue after 12 drops (your number will differ). At that point the acid has been neutralised and a little extra base is present. Add one more drop of lemon juice, and the colour moves back towards red. Each step shows the same idea: the colour tells you which one is in excess.
Example 4: the ant bite. An ant injects formic acid, which is acidic, so the remedy must be basic. Moist baking soda works; vinegar would not.
What mistakes do students make with acids and bases?
Research on this topic finds that misconceptions are common and stubborn. In one study of a four-week unit on acids and bases, Demircioglu, Ayas and Demircioglu (2005) gave one group worksheets built on "conceptual conflict", where students first predict and then face a result that challenges them. That group scored higher and held fewer misconceptions than a group taught the usual way. The chapter's own "predict, then test" tables work on the same idea, so fill in the prediction column honestly before you test.
The mistakes to watch for:
"Red litmus turning red means acid." Red litmus stays red in an acid. Only blue litmus changes colour.
"Everything bitter is a base." The book's example is karela: bitter but not basic. Sour foods do tend to contain acids, but never taste unknown substances.
"Neutral means pure water." Sugar solution and salt solution are neutral too.
"Turmeric shows acids." It shows bases only.
"Lime water is lemon juice." Lime water is calcium hydroxide solution, made from chuna. It is basic.
How does this connect to later classes?
You will meet acids and bases again in Class 10, with pH and the names of laboratory indicators. Our Class 10 guide to acids, bases and salts shows where this chapter leads. For now, the habit that matters is reading an indicator result carefully and saying what it does and does not prove.
A one-week practice plan
Day 1: Make the four-line indicator list above from memory. Check it against the chapter's "In a Nutshell".
Day 2: With an adult, make red rose or turmeric indicator at home and test five kitchen liquids. Our post on kitchen science experiments has tips on doing this safely.
Day 3: Write three "unlabelled bottle" puzzles like Example 1 for a friend, then swap.
Day 4: Answer the chapter questions from memory, then check. Mark each mistake as "wrong indicator rule" or "wrong reasoning".
Day 5: Explain neutralisation to a parent using the ant bite.
For parents: the chapter's activities use safe kitchen items. Supervise anything involving hot water, and keep lime and washing powder away from eyes.
If the idea of an indicator still feels like a list to memorise, look it up as a concept. The Fermi Concept Library has every concept in chemistry with its own lesson, so you can read about indicators and neutralisation and then come back to the chapter.
This evening, test three kitchen liquids on turmeric paper and predict each result before you look.
