Question

An element A is soft and can be cut with a knife. This is very reactive to air and cannot be kept open in air. It reacts vigorously with water. Identify the element from the following

• A. Mg
• B. Na
• C. P
• D. Ca

Answer 1

The correct option is B Na

Hint: Generally alkali metals are soft and can be cut with a knife and are highly reactive.
Step 1: Alkali metals like sodium, potassium, etc are soft and can be easily cut with a knife. They are highly reactive metals and undergo combustion when exposed to air. These metals react vigorously even with cold water. So, they are stored in kerosene oil.

Step 2:

A. Magnesium (Mg) is a soft metal that can be cut with a knife but it does not react significantly with cold water.

B. Sodium (Na) is a soft alkali metal that can be cut with a knife and reacts vigorously with cold water.

C. Phosphorous (P) is a non-metal and its allotropes can be cut with a knife and reacts with water at high temperature in steam form.

D. Calcium (Ca) is a soft metal that can be cut with a knife but not as easily as sodium. Even it does not react so vigorously with cold water and is not so reactive like sodium in air.
Step 3: Sodium is a soft metal and highly reactive because of the loosely bonded single valence electrons. The reaction of sodium with air and water are as follows:
$4Na\left(s\right)+O_2\left(g\right)\to 2N{a}_{2}O\left(s\right)$ ----- with air
$2Na\left(s\right)+H_{2}O\left(l\right)\to 2NaOH\left(aq\right)+H_2\left(g\right)$ ---- with water
Final step: So, the correct option is (B) Na.