natural-indicators MCQs for UPSC Prelims

26 practice questions on natural-indicators from the Exploring Substances: Acidic, Basic, and Neutral section of the UPSC Prelims syllabus. 26 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

8 Easy 13 Medium 5 Hard

Sample questions

Q1
easy
Turmeric paper turns red when a drop of
  1. A plain distilled water falls on it
  2. B an acidic solution falls on it
  3. C a basic solution falls on it
  4. D a neutral solution falls on it
Show answer and explanation

Correct answer: C - a basic solution falls on it

Turmeric changes from yellow to red only with a base, which is the whole point of using it as an indicator. An acid leaves turmeric yellow, so the second choice is wrong. A neutral solution and distilled water also leave it yellow, so neither of the last two produces a change.
Q2
medium
A few drops of china rose extract are added to a glass of lime water. What colour is expected?
  1. A Green
  2. B Yellow
  3. C Orange
  4. D Magenta
Show answer and explanation

Correct answer: A - Green

Lime water is basic, and china rose extract turns green in a base. Magenta would appear only if the liquid were acidic, which lime water is not. Yellow and orange are not colours this indicator gives with either an acid or a base.
Q3
easy
Litmus, the best known laboratory indicator, is obtained from
  1. A turmeric roots
  2. B tea leaves
  3. C lichens
  4. D rose petals
Show answer and explanation

Correct answer: C - lichens

Litmus is a dye taken from lichens, which are small growths found on rocks and tree bark. Rose petals give a different natural indicator, and turmeric roots give the yellow turmeric indicator. Tea leaves are not used to prepare litmus at all.
Q4
easy
Indicators are useful in the laboratory because they
  1. A show by a change of colour whether a solution is acidic or basic
  2. B make any solution safe to drink
  3. C warm a solution so that it reacts faster
  4. D remove all the acid from a solution
Show answer and explanation

Correct answer: A - show by a change of colour whether a solution is acidic or basic

An indicator answers a question about a solution by changing colour, which is why it is used. It does nothing to make a solution safe to drink. It does not take acid out of a solution. It also does not heat anything, so the last choice describes something an indicator never does.
Q5
medium
An unknown solution turns blue litmus paper red and leaves red litmus paper unchanged. The solution is
  1. A neutral
  2. B an indicator
  3. C acidic
  4. D basic
Show answer and explanation

Correct answer: C - acidic

Turning blue litmus red is the test result for an acid, so the solution is acidic. A base would have turned the red paper blue instead. A neutral solution would have left both papers unchanged. Being an indicator is not a category the litmus test can report.
Q6
easy
An acidic solution turns
  1. A blue litmus paper green
  2. B red litmus paper blue
  3. C red litmus paper yellow
  4. D blue litmus paper red
Show answer and explanation

Correct answer: D - blue litmus paper red

Acids turn blue litmus red, and this is the standard test for an acid. Turning red litmus blue is what a base does, so the second choice reverses the rule. Litmus does not turn green or yellow with acids, so the last two describe changes that are never seen.
Q7
medium
A solution leaves blue litmus blue, leaves red litmus red, and leaves turmeric paper yellow. The solution is
  1. A a strong base
  2. B basic
  3. C acidic
  4. D neutral
Show answer and explanation

Correct answer: D - neutral

No indicator has changed, and that is exactly how a neutral solution behaves. An acid would have turned the blue paper red. A base would have turned the red paper blue and the turmeric red. A strong base would have shown those changes even more clearly, so it is ruled out too.
Q8
easy
An extract prepared from china rose (hibiscus) petals is used in the laboratory as
  1. A a strong acid
  2. B a cleaning agent
  3. C a fuel
  4. D a natural indicator
Show answer and explanation

Correct answer: D - a natural indicator

The petal extract changes colour with acids and bases, so it works as a natural indicator. It is not an acid itself, and it is far too weak to clean anything. It is certainly not burnt as a fuel, so none of the last three describes its use.

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More Exploring Substances: Acidic, Basic, and Neutral topics

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