CBSE Class 7 Science Chapter 02 Exploring Substances Acidic, Basic, And Neutral MCQs Set 01

Answer: (A) An acid-base indicator reacting with a substance already on the paper

Explanation:
The chapter later reveals that the trick likely involved turmeric paste applied to the paper (an indicator) and a basic solution like soap used for writing, which turns yellow turmeric red when sprayed.

Answer: (B) It changes to different colours depending on whether a substance is acidic or basic

Explanation:
Litmus turns blue litmus paper red with acids and red litmus paper blue with bases, so its colour-changing behaviour is what makes it useful for distinguishing the two types of substances.

Answer: (C) Lichens

Explanation:
Litmus comes from lichens, organisms formed by a fungus and an alga living together, unlike the other options which are separate examples of natural indicators mentioned in the chapter.

Answer: (C) They are neutral

Explanation:
Substances that do not affect either litmus paper are classified as neutral, since they are neither acidic nor basic.

Answer: (C) It is basic in nature

Explanation:
Basic substances generally feel slippery or soapy to touch, which is one of the properties described for classifying bases alongside their effect on litmus.

Answer: (B) Lemon juice turns it red; soap solution turns it green

Explanation:
As shown in the test tube demonstration, the acidic lemon juice gave a red colour while the basic soap solution gave a green colour with the red rose extract.

Answer: (B) It cannot tell apart acidic and neutral substances since only bases change its colour

Explanation:
Turmeric paper turns red only in the presence of basic substances; since both acids and neutral substances leave it unchanged, it fails to distinguish between the two.

Answer: (C) A basic solution like soap or baking soda solution

Explanation:
Since turmeric paper only changes colour (to red) when a basic substance touches it, Ashwin must have written with a basic solution such as soap or baking soda solution for the message to appear.

Answer: (B) Olfactory indicators

Explanation:
The chapter names such odour-changing substances, like chopped onion strips reacting to tamarind water or baking soda, as olfactory indicators.

Answer: (C) It changed from red to blue, showing neutralisation of the acid

Explanation:
Adding blue litmus to the diluted lemon juice first turned it red; as lime water was added drop by drop, the acid was gradually neutralised until the solution turned blue.

Answer: (B) Salt and water

Explanation:
The neutralisation reaction is summarised as Acid + Base → Salt + Water + Heat, so salt and water are the two substances produced along with released heat.

Answer: (B) Ant bites inject an acidic liquid, and baking soda being basic neutralises it

Explanation:
Ant bites release formic acid into the skin; since baking soda is a base, rubbing it on the bite neutralises the acid and eases the stinging pain.

Answer: (B) Adding lime, a basic substance, to the soil

Explanation:
Since the soil has turned acidic, treating it with lime (a base) neutralises the excess acidity, similar to how lime water was used to neutralise lemon juice in the classroom activity.

Answer: (B) Acharya Prafulla Chandra Ray

Explanation:
P.C. Ray earned his doctorate in chemistry in the UK, returned to advance Indian scientific research, founded the country's first pharmaceutical company in 1901, and also promoted teaching in the mother tongue.

Answer: (A) A fungus and an alga growing together

Explanation:
Lichens form when a fungus and an alga live together, and they are typically found on rocks and trees in areas with plenty of rainfall and clean air.

Answer: (B) Vinegar is acidic and reacts with the eggshell, while soap solution is basic and does not cause the same reaction

Explanation:
Vinegar is an acid, and its reaction with the eggshell produces gas bubbles; soap solution, being basic in nature, does not trigger this same reaction with the shell's material.

Answer: (B) Blue

Explanation:
Hydrangeas growing in acidic soil produce blue flowers, while the same plant produces pink or red blooms when grown in basic soil.

Answer: (B) They must be crushed using a mortar and pestle

Explanation:
After washing the collected petals, they are crushed with a mortar and pestle before being placed in a tumbler and covered with hot water to draw out the colour.

Answer: (C) Tartaric acid

Explanation:
The chapter's diagram links tartaric acid with tamarind, while citric acid is linked to citrus fruits, ascorbic and citric acid to amla, and citric and oxalic acid to tomato.

Answer: (B) A bitter taste alone is not a reliable way to confirm a substance is basic

Explanation:
The bitter gourd example warns against assuming bitterness always means a substance is basic — bases generally taste bitter, but the reverse is not necessarily true.

Answer: (B) Because the base and acid cancel out each other's effect, leaving the mixture neither acidic nor basic

Explanation:
When enough base is added to an acidic solution, a neutralisation reaction occurs, forming salt and water and releasing heat, so the resulting mixture is no longer acidic or basic — this is why the colour stabilises.

Answer: (B) Adding organic matter such as manure and composted leaves

Explanation:
Basic soil is treated by adding organic matter like manure or composted leaves, since decomposing organic matter releases acids that neutralise the soil's basic nature.
• Acidic soil, on the other hand, is treated with lime.
• This is the opposite remedy from what is used for acidic soil.

Answer: (B) Treating the waste with a basic substance before it is released into the lake

Explanation:
The chapter suggests neutralising acidic factory waste with a basic substance before it reaches the lake, protecting the fish from the harmful acidic effect.

Answer: (C) Using the mother tongue as the medium of instruction in schools

Explanation:
P.C. Ray was not just a scientist but also a social reformer who pushed for the mother tongue to be used as the language of teaching in educational institutions.