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Exploring Substances: Acidic, Basic and Neutral: questions with answers

Class 7 Science, Chapter 2 (NCERT). 15 questions by concept. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

How acids and bases behave: sour, bitter, slippery, and the limits of taste

Question 1

Bitter gourd, or karela, has a strongly bitter taste. Aditi concludes from this that it is basic in nature. Is she right?

  1. (a)Yes, because bases are the only substances that taste bitter.
  2. (b)No, because a vegetable cannot have any nature until it is juiced.
  3. (c)No, because a bitter taste does not by itself mean a base is present.
  4. (d)Yes, because anything with a bitter taste contains a base.
Show answer

(c) No, because a bitter taste does not by itself mean a base is present.

Bases generally taste bitter. But everything that tastes bitter may not contain a base. Bitter gourd (karela) is bitter in taste and yet it is not basic in nature.

Question 2

Baking soda solution feels slippery despite containing no added soap. Which known property accounts for this observation?

  1. (a)Soap as the only source of slipperiness
  2. (b)Cleanliness as the source of slipperiness
  3. (c)The slippery feel common to basic solutions
  4. (d)An acidic nature
Show answer

(c) The slippery feel common to basic solutions

Baking soda solution is basic and can feel slippery without containing soap. Slipperiness does not establish cleanliness.

Red rose extract: red in acidic, green in basic

Question 3

What happens to red rose extract when a neutral solution is added?

  1. (a)It separates into two layers
  2. (b)A distinct third colour appears
  3. (c)Its original colour remains
  4. (d)It becomes colourless
Show answer

(c) Its original colour remains

Neutral solutions leave the rose extract’s colour unchanged. A distinct new neutral colour is unnecessary.

Question 4

A sample is known to have turned blue litmus paper red. What will red rose extract do when it is added to the same sample?

  1. (a)Turn green, because the extract reverses litmus.
  2. (b)Stay unchanged, because the extract adds nothing new.
  3. (c)Turn green, because the sample is acidic.
  4. (d)Turn red, because the sample is acidic.
Show answer

(d) Turn red, because the sample is acidic.

Turning blue litmus red makes the sample acidic, and red rose extract gives a red colour in an acidic solution. The two indicators agree rather than reverse each other, and an unchanged extract is what a neutral sample would give.

Litmus: what it is and what its two colour changes mean

Question 5

An unknown sample leaves blue litmus unchanged. Which possibilities remain?

  1. (a)Acidic only
  2. (b)Basic only
  3. (c)Acidic or basic, but not neutral
  4. (d)Basic or neutral
Show answer

(d) Basic or neutral

Basic and neutral samples both leave blue litmus blue. Red litmus distinguishes them.

Question 6

A small drop of blue litmus solution turns red in a sample. What does this indicate?

  1. (a)The sample was already acidic
  2. (b)The sample was neutral
  3. (c)Litmus made the sample acidic
  4. (d)The sample was basic
Show answer

(a) The sample was already acidic

A small testing amount of litmus reports the sample’s existing acidic nature. Acids turn blue litmus red.

Sorting substances into acidic, basic and neutral

Question 7

Which set of natures occurs among household vinegar, soap solution and sugar solution?

  1. (a)Acidic only
  2. (b)Acidic, basic and neutral
  3. (c)Basic only
  4. (d)Neutral only
Show answer

(b) Acidic, basic and neutral

Vinegar is acidic, soap solution is basic and sugar solution is neutral. Household origin does not determine a substance’s nature.

Question 8

Rohan tests a sugar solution and finds that it changes neither the red litmus paper nor the blue litmus paper. He calls it a very weak acid. What is the nature of the sugar solution?

  1. (a)It is neutral, because it is neither acidic nor basic.
  2. (b)It is basic, because every sample must be either acidic or basic.
  3. (c)It is acidic, because a neutral sample is really an acid too weak to be shown.
  4. (d)No nature can be given, because papers that stay unchanged mean the test did not work.
Show answer

(a) It is neutral, because it is neither acidic nor basic.

A sugar solution changes neither litmus paper, and such substances are neutral. Neutral means neither acidic nor basic at all. Rohan keeps the sample on an acid scale, which is exactly what the word neutral sets aside.

Turmeric paper: red in basic, unchanged in both acidic and neutral

Question 9

Dried turmeric strips are dipped in vinegar and in soap solution. What colours do they show, respectively?

  1. (a)Yellow; yellow
  2. (b)Red; red
  3. (c)Red; yellow
  4. (d)Yellow; red
Show answer

(d) Yellow; red

Vinegar is acidic and soap solution is basic. Yellow turmeric stays yellow in an acid and turns red in a base.

Preparing a natural indicator from a flower

Question 10

Which material can supply a natural colour indicator through a method similar to preparing red rose extract?

  1. (a)No material other than red rose
  2. (b)Any dried leaf
  3. (c)Any strongly scented plant
  4. (d)Purple cabbage
Show answer

(d) Purple cabbage

Purple cabbage contains colouring substances that respond to acidic and basic conditions. A strong smell or drying alone does not establish this property.

Neutralisation: what an acid and a base make together

Question 11

What completes the chapter’s word equation: Acid + Base → ?

  1. (a)Water + Heat; no salt
  2. (b)Salt + Heat; no water
  3. (c)Salt + Water + Heat
  4. (d)Salt + Water; heat absorbed
Show answer

(c) Salt + Water + Heat

The chapter’s neutralisation equation includes salt, water and released heat. Omitting heat does not mean that heat is absorbed.

Olfactory indicators: when the clue is a smell, not a colour

Question 12

Onion strips are prepared, their odour is checked, drops are added, and the odour is checked again. What is being compared?

  1. (a)The odour of the tamarind water against that of the baking soda.
  2. (b)The odour of the closed container against that of the open room.
  3. (c)The odour of a strip before the drops against its odour after.
  4. (d)The odour of a fresh onion against that of one kept overnight.
Show answer

(c) The odour of a strip before the drops against its odour after.

Two strips are taken and their odour is checked. Then a few drops of tamarind water go on one strip and a few drops of baking soda solution on the other. The odour is checked again. The comparison is on each strip, before against after.

Neutralisation put to work: ant bites, soil and factory waste

Question 13

Acidic factory waste requires neutralisation. Which treatment plan protects the lake from its acidity?

  1. (a)Addition of a base after discharge
  2. (b)Controlled addition of a suitable base before discharge
  3. (c)Dilution by lake water after discharge
  4. (d)Addition of another acid before discharge
Show answer

(b) Controlled addition of a suitable base before discharge

A suitable base neutralises the acid. Treatment must happen before discharge so aquatic life is not exposed to acidic waste.

The secret message: choosing a writing liquid and a spray to match

Question 14

What produces red lettering when turmeric solution is sprayed over dried soap-solution writing?

  1. (a)Turmeric’s universal response to every invisible ink
  2. (b)Paper wetting by any spray
  3. (c)Turmeric’s colour response to a base
  4. (d)Development of hidden dye in any dried ink
Show answer

(c) Turmeric’s colour response to a base

Basic soap changes turmeric from yellow to red. The writing needs no hidden dye, and the response depends on the writing liquid’s nature.

P.C. Ray and the writing of India's chemical history

Question 15

How far back did Indian chemistry extend before P.C. Ray, as shown by his historical writings?

  1. (a)Only to the year 1901
  2. (b)To ancient Indian scientists
  3. (c)Only to the generation immediately before Ray
  4. (d)Only to Ray’s generation
Show answer

(b) To ancient Indian scientists

Ray described achievements of ancient Indian scientists. His title refers to modern Indian chemistry, not the beginning of all chemistry in India.

Common misconception in Exploring Substances: Acidic, Basic and Neutral

Open full misconception guide →

Does one unchanged litmus strip prove neutrality?

Wrong

If blue litmus stays blue, the liquid must be neutral.

Why it feels right. I know how an acid changes blue litmus and assume every nonacid is neutral.

Right

Use both colours of litmus to distinguish acidic, basic and neutral samples in this test.

Worked example

Basic soap
Blue stays blue→Red turns blue
Neutral water
Blue stays blue→Red stays red
Blue litmus alone looks the same in both samples. Red litmus separates the cases.
  1. In a basic soap solution, blue litmus stays blue but red litmus turns blue.
  2. In a neutral water sample both strips keep their original colours.