Question

Which among the following is true, when calcium is treated with cold water?

• A. Calcium does not react with cold water.
• B. Hydrogen gas evolved catches fire.
• C. Hydrogen gas evolved does not catch fire.
• D. Calcium floats on the surface of the water.

Answer 1

Answer: (C), (D)

The correct options are
C Hydrogen gas evolved does not catch fire.
D Calcium floats on the surface of the water.

Calcium does not undergo violent reaction with water. On reaction with water, calcium starts floating. This is because the bubbles of hydrogen gas evolved during the reaction stick to its surface and lifts it up. The reaction involved in this process is:

$\mathrm{Ca}\left(s\right)+\; 2H_{2}O\left(l\right)\to \mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)+H_2\left(g\right)$

Even though it is an exothermic reaction, the heat evolved is not sufficient for hydrogen to catch fire.