Question

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

Reason: Calcium reacts with cold water at room temperature.

• A. Both assertion and reason are correct and reason is the correct explanation for assertion.
• B. Both assertion and reason are correct but reason is not the correct explanation for assertion.
• C. Assertion is correct but reason is incorrect.
• D. Both assertion and reason are incorrect.

Answer 1

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:-

$\underset{\mathrm{Calcium}}{\mathrm{Ca}\left(\mathrm{s}\right)}+\underset{\mathrm{Water}}{2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{\mathrm{Calcium}\mathrm{hydroxide}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}+\underset{\mathrm{Hydrogen}\mathrm{gas}}{{\mathrm{H}}_2\left(\mathrm{g}\right)}$

• 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.