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Question

Assertion: On reacting with water, Calcium starts floating over water.

Reason: Calcium reacts with cold water at room temperature.


  1. Both assertion and reason are correct and reason is the correct explanation for assertion.

  2. Both assertion and reason are correct but reason is not the correct explanation for assertion.

  3. Assertion is correct but reason is incorrect.

  4. Both assertion and reason are incorrect.

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Solution

The correct option is C

Assertion is correct but reason is incorrect.


Explanation for correct option:

Assertion:-

  • Calcium is an alkaline earth metal with atomic number 20 and it forms Calcium hydroxide and hydrogen gas when reacted with water.
  • This reaction is very slow
  • The complete reaction is:-

Ca(s)Calcium+2H2O(l)WaterCa(OH)2(aq)Calciumhydroxide+H2(g)Hydrogengas

  • Here the heat released is very small, and it is not enough for the Hydrogen that is formed to catch fire.
  • Instead of this, the calcium starts floating because the bubbles of the Hydrogen gas which is formed stick to the surface of the water.
  • Thus the assertion is true.

Explanation for reason:-

  • Calcium belongs to group 2, and group 2 elements don't have high solubility in water as the group 1 element have.
  • This is the reason why the reaction between calcium and water goes faster in warm water rather than in cold water.
  • This reaction is not as vigorous as the group 1 elements like Lithium, Sodium, and Potassium.
  • Thus the reason is false.

Therefore, option (C) Assertion is correct but reason is incorrect is the correct answer.


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