Question

Assertion: A piece of sodium metal catches fire when thrown in water.

Reason: Sodium is a very reactive metal.

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

Answer 1

The correct option is B

If both assertion and reason are correct but the reason is not the correct explanation for assertion.

The explanation for the correct option.

(B) If both assertion and reason are correct but the reason is not the correct explanation for assertion.

The explanation for the assertion:

The reaction of sodium with water.

1. Sodium$\mathbf{\left(}\mathbf{Na}\mathbf{\right)}$ reacts with water$\mathbf{\left(}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\right)}$ to form sodium hydroxide$\mathbf{\left(}\mathbf{NaOH}\mathbf{\right)}$ and hydrogen$\mathbf{\left(}{\mathbf{H}}_{\mathbf{2}}\mathbf{\right)}$ gas.
2. $2\mathrm{Na}\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2\mathrm{NaOH}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right);∆\mathrm{H}=-369\mathrm{kJ}$
3. In this reaction, sodium does not burn but rather hydrogen gas(highly combustible) produced burns due to the heat released in the reaction.

Hence, the assertion is correct.

The explanation for the reason statement.

Sodium is a very reactive metal.

1. Sodium has 1 electron in its outer shell. It has a high tendency to lose this electron to reach the stable configuration of argon$\mathbf{\left(}\mathbf{Ar}\mathbf{\right)}$.
2. This tendency to donate electrons is why sodium is very reactive and reacts with almost any element.

Hence, the reason is correct but does not explain the assertion.

The correct reason for the assertion

The hydrogen produced in the reaction of sodium with water easily catches fire due to the heat from the reaction as it is a highly combustible gas.

Hence, the correct option is (B).