acids-bases-and-neutral-substances MCQs for UPSC Prelims

26 practice questions on acids-bases-and-neutral-substances 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
medium

Lime water, milk of magnesia and a solution of washing soda are all basic. All three would therefore

  1. A leave both litmus papers unchanged
  2. B turn red litmus paper blue
  3. C turn blue litmus paper red
  4. D taste sour
Show answer and explanation

Correct answer: B - turn red litmus paper blue

Bases turn red litmus blue, so all three would give that change. Turning blue litmus red is what an acid does. Leaving both papers unchanged is the behaviour of a neutral substance. A sour taste belongs to acids, not to these three.

Q2
medium

Statement I: A neutral solution changes the colour of neither litmus paper. Statement II: A neutral solution turns turmeric paper red. Which of these is correct?

  1. A Both statements are correct
  2. B Only Statement II is correct
  3. C Neither statement is correct
  4. D Only Statement I is correct
Show answer and explanation

Correct answer: D - Only Statement I is correct

Statement I is correct, since leaving both litmus papers unchanged is exactly how a neutral solution behaves. Statement II is wrong, because turmeric turns red only with a base and stays yellow in a neutral solution. So only the first statement stands.

Q3
medium

Why is it dangerous to taste an unknown liquid in order to find out whether it is acidic?

  1. A It may spoil the taste of the next meal
  2. B It may change colour once it is inside the mouth
  3. C It may be corrosive or poisonous and can injure the mouth and throat
  4. D It may make the liquid useless for the rest of the experiment
Show answer and explanation

Correct answer: C - It may be corrosive or poisonous and can injure the mouth and throat

Laboratory acids and bases can burn the soft lining of the mouth and throat, and some substances are poisonous, so tasting risks serious injury. Spoiling a meal or wasting a sample are trivial worries beside that. Whether the liquid changes colour in the mouth is not the danger, and it would not be safe even if it did not.

Q4
medium

Which of these solutions is neutral?

  1. A Lemon juice
  2. B Soap solution
  3. C Vinegar
  4. D Common salt dissolved in water
Show answer and explanation

Correct answer: D - Common salt dissolved in water

Common salt solution is neither sour nor soapy, and it leaves litmus unchanged, so it is neutral. Lemon juice and vinegar are both acidic and taste sour. Soap solution is basic and feels slippery, so it is not neutral either.

Q5
easy

Vinegar used in the kitchen is

  1. A neutral
  2. B an indicator
  3. C basic
  4. D acidic
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Correct answer: D - acidic

Vinegar contains an acid, which gives it a sharp, sour taste. A basic liquid would taste bitter and feel soapy. A neutral liquid would leave litmus unchanged, but vinegar turns blue litmus red. Vinegar does not change colour to reveal other substances, so it is not an indicator.

Q6
medium

Amla, tamarind and unripe mango share which property?

  1. A They all contain acids
  2. B They all feel soapy
  3. C They are all neutral
  4. D They all contain bases
Show answer and explanation

Correct answer: A - They all contain acids

All three taste sour, and sourness comes from the acids they contain. If they held bases they would taste bitter instead. A neutral food would have no sour taste at all. A soapy feel belongs to basic substances, not to these fruits.

Q7
easy

Which of these kitchen items is acidic?

  1. A Soap solution
  2. B Lime water
  3. C Lemon juice
  4. D Baking soda solution
Show answer and explanation

Correct answer: C - Lemon juice

Lemon juice contains an acid, which is why it tastes sour. Baking soda dissolved in water is basic and tastes bitter. Soap solution is basic and feels slippery. Lime water is also basic, so none of the last three is acidic.

Q8
hard

Assertion (A): A laboratory acid must never be tasted, even though acids are known for their sour taste. Reason (R): Every acid found in a laboratory is safe to taste in a small amount.

  1. A Both A and R are true, and R is the correct explanation of A
  2. B Both A and R are true, but R is not the correct explanation of A
  3. C A is true, but R is false
  4. D A is false, but R is true
Show answer and explanation

Correct answer: C - A is true, but R is false

A is true, because laboratory acids can be strong enough to burn the mouth and throat. R is false, since no laboratory acid may be tasted, however small the amount. As R is false, the first two choices are ruled out, and the last choice fails because A is true.

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