Question

Which of the following reactions will result in the formation dihydrogen gas?

• A. A reaction of potassium with cold water.
• B. A reaction of sodium with cold water.
• C. A reaction of sodium with oxygen.
• D. A reaction of potassium with oxygen.

Answer 1

Answer: (A), (B)

The correct options are
A

A reaction of potassium with cold water.

B

A reaction of sodium with cold water.

Highly reactive metals like potassium, sodium, and calcium, when exposed to cold water, liberate dihydrogen gas along with energy.
$2K\left(s\right)+H_{2}O\left(l\right)\to 2KOH\left(aq\right)+H_2\left(g\right)$
$2Na\left(s\right)+H_{2}O\left(l\right)\to 2NaOH\left(aq\right)+H_2\left(g\right)$

When these reactive metals are exposed to oxygen gas, they form corresponding metal oxide.
$4K\left(s\right)+O_2\left(g\right)\to 2K_{2}O\left(s\right)$
$4Na\left(s\right)+O_2\left(g\right)\to 2N{a}_{2}O\left(s\right)$