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Class 7 Science: Exploring Substances: Acidic, Basic, and Neutral, 8 practice questions with answers

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These are 8 practice questions on the second chapter of the new NCERT Class 7 Science textbook, Curiosity, called Exploring Substances: Acidic, Basic, and Neutral. Each has a short answer and the textbook page it comes from. They cover litmus, taste and touch, choosing an indicator, what turmeric paper can and cannot tell, other natural indicators, neutralisation, watching neutralisation with turmeric, and how an antacid works. For the same topic at board level, see our Class 10 Acids, Bases and Salts guide. Try each question before you read the answer.

These questions are our own, drafted with AI help. Every fact was checked against the page it cites by a test script, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.

What the chapter says you need

Facts from the NCERT textbook (Curiosity, Grade 7, Chapter 2, printed pages 7 to 22):

Indicators

Question 1. (a) What is litmus, and where does it come from? (b) Why is it called an acid-base indicator? (c) What does each kind of litmus paper show?

Answer: (a) a natural substance from lichens; (b) its colour depends on whether a solution is acidic or basic; (c) blue turns red in acids, red turns blue in bases. (a) Litmus is natural: it is made from lichens, which are formed by a fungus and an alga living together. It is sold as a solution and as blue and red paper strips. (b) Litmus takes one colour in an acid and another in a base, so it is an acid-base indicator. (c) A substance that turns blue litmus red is acidic; one that turns red litmus blue is basic. A substance that changes neither paper is neutral. (Litmus as an indicator, page 10.)

Question 2. A friend says, "Anything that tastes bitter must be basic." (a) Is that right? (b) How do acidic and basic substances usually taste and feel? (c) What rule about tasting does the chapter give?

Answer: (a) no; (b) acidic ones taste sour, basic ones feel slippery and usually taste bitter; (c) taste nothing unless your teacher asks, and never anything unknown. (a) Bases generally taste bitter, but bitterness does not prove a substance is basic: the chapter points out that karela (bitter gourd) tastes bitter and yet is not basic. (b) A sour taste usually points to an acid. Basic substances are generally soapy or slippery to touch. (c) Taste nothing unless your teacher asks you to, and never put an unknown substance in your mouth. Taste is not a safe test; an indicator is. (Taste and touch, page 11.)

Question 3. You have only one of these at a time: blue litmus paper, red rose extract, or turmeric paper. Which one, used on its own, can sort a liquid into acidic, basic or neutral in a single test? What can each of the other two tell you?

Answer: red rose extract; blue litmus tells only acidic or not, turmeric only basic or not. Red rose extract gives red in an acidic solution, green in a basic one, and no change in a neutral one, so one test shows all three. Blue litmus turns red only for an acid: if it stays blue, the liquid could be basic or neutral. Turmeric turns red only for a base: if it stays yellow, the liquid could be acidic or neutral. (If you make the red rose extract yourself, use petals that have fallen, not plucked flowers, and do the hot-water step with an adult.) (Litmus, page 10; preparing the red rose extract, page 12; its colours, page 13; turmeric, page 15.)

Question 4. A drop of an unknown liquid is put on turmeric paper, and the paper stays yellow. (a) What can you say about the liquid, and what can you not say? (b) Which test would settle it?

Answer: (a) it is not basic, but it could be acidic or neutral; (b) a drop on blue litmus paper. (a) Turmeric turns red in basic solutions but stays unchanged in acidic and neutral ones. No change rules out a base, but it cannot tell an acid from a neutral liquid. (b) Put a drop on blue litmus paper: if it turns red, the liquid is acidic; if nothing changes, it is neutral. (Red rose extract would also work: red for an acid, no change for a neutral liquid.) (Turmeric as an indicator, page 15; litmus, page 10; red rose, page 13.)

More indicators, and neutralisation

Question 5. (a) Besides litmus, red rose and turmeric, name four plant materials that the chapter says can also act as acid-base indicators. (b) A student makes an extract from one of them. Using liquids whose nature she already knows, how can she check whether her extract really is an acid-base indicator?

Answer: (a) beetroot, purple cabbage, Indian blackberry (jamun) and red hibiscus (gudhal); (b) add it to a known acidic liquid and a known basic liquid and see whether it shows two different colours. (a) The chapter suggests trying extracts of beetroot, purple cabbage, jamun and red hibiscus, which can also act as acid-base indicators. (b) An acid-base indicator is a substance that shows one colour in acidic solutions and a different colour in basic ones. So she can add a few drops of her extract to a liquid she knows is acidic, such as lemon juice, and to one she knows is basic, such as soap solution (and, as a check, to a neutral one such as sugar solution). If the colour in the acid differs from the colour in the base, the extract works as an indicator. (Other natural indicators, page 14; what an indicator is, pages 10 and 19; the natures of these liquids, page 10.)

Question 6. (a) Write the word equation for a neutralisation reaction. (b) Is heat taken in or given out? (c) Why does the chapter say the acid and the base must be mixed "in sufficient quantity" for the result to be neither acidic nor basic?

Answer: (a) acid + base → salt + water + heat; (b) given out; (c) if too little of one is added, some of the other is left over. (a) When an acid and a base react, a salt and water are formed. (b) Heat is released (given out) in the reaction. (c) The solution becomes neither acidic nor basic only when enough base has been added to neutralise all the acid (or enough acid for all the base). With too little, the solution is still acidic (or basic). (Neutralisation, pages 17 and 18.)

In daily life

Question 7. A little soap solution is mixed with a few drops of turmeric solution, and the mixture turns red. Then lemon juice is added drop by drop. (a) Why did the mixture turn red at first? (b) What happens to the red colour as enough lemon juice is added, and why? (c) If still more lemon juice is added after that, will the turmeric show any new colour?

Answer: (a) soap solution is basic, and turmeric turns red in a base; (b) it goes back to yellow, because the acid neutralises the base; (c) no, it stays yellow. (a) Turmeric's yellow colour turns red in basic solutions, and soap solution is basic. (b) Lemon juice is acidic. As it is added, it neutralises the base, and once enough has been added the mixture is no longer basic, so the turmeric is yellow again. (c) No: turmeric stays unchanged in acidic and neutral solutions alike, so extra lemon juice does not change it further. To see the mixture become acidic you would need an indicator such as blue litmus or red rose extract. (Turmeric, page 15; neutralisation, pages 17 and 18; the natures of soap solution and lemon juice, page 10.)

Question 8. After a very heavy meal, someone has a burning feeling because there is too much acid in the stomach, and a doctor suggests an antacid tablet. (a) Is an antacid acidic or basic? (b) How does it bring relief? (c) Why is swallowing a large number of tablets not a good idea?

Answer: (a) basic, a mild base; (b) it neutralises the extra acid; (c) too much base would leave the stomach basic instead. (a) An antacid is a mild base. (The chapter does not name antacids; you meet them again in Class 10.) (b) A base cancels the effect of an acid: the two react to give a salt and water, and the extra acid that caused the burning is used up. This is neutralisation, the same reaction the chapter describes. (c) Neutralisation leaves a result that is neither acidic nor basic only when the amounts match, as in Question 6. A small dose deals with the extra acid; far more base than is needed would push things too far the other way. Medicines should be taken only as a doctor or the packet advises. (Neutralisation, pages 17 and 18.)

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Sources

All opened and read on 2 November 2026. Page numbers are the printed page numbers of the textbook; printed page = PDF page + 6 for this chapter.

The facts are from the textbook pages above. The questions, answers and checks are our own. This guide is general information; follow your teacher and the textbook.

Practise with the worksheet: Acidic, Basic and Neutral, Class 7 worksheet. Printable, in English, Hindi and Marathi, with an answer key.