CDS 2026 (I) General Knowledge, question 27

Topic: Chemistry

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Q27

Consider the following statements :

  1. The temporary hardness in water is due to the presence of bicarbonates of calcium.

  2. The permanent hardness in water is due to the presence of soluble chlorides and sulphates of calcium and magnesium.

  3. The permanent hardness in water is removed by adding Na2CO3Na_{2}CO_{3} to water.

Which of the statements given above are correct ?

  1. (a) 1 and 2 only
  2. (b) 2 and 3 only
  3. (c) 1 and 3 only
  4. (d) 1, 2 and 3Correct answer

Solution.

Answer: (d) 1, 2 and 3

  1. All three statements are correct.
  2. Calcium bicarbonate contributes to temporary hardness; calcium and magnesium chlorides and sulphates cause permanent hardness.
  3. Sodium carbonate removes the hardness ions by forming poorly soluble carbonates.
  4. Magnesium bicarbonate can also contribute to temporary hardness; statement 1 does not say calcium is the only cause.