Question

When sodium is dropped in a small amount of water it catches fire. Which one of the following gas is released in this process?
A. $Na$
B. ${H_2}O$
C. ${H_2}$
D. $NaOH$

Answer 1

Hint: Sodium is a very highly reactive metal and on its addition to water, it splits the water molecules to form sodium hydroxide. The liberated gas is what catches fire, and is identified by a characteristic ‘pop’ sound.

Complete step by step answer:
Sodium is a very highly reactive metal belonging to the alkali metal group (group one) of the periodic table. Its reaction with water is highly vigorous and exothermic, releasing a large amount of heat. When added to water, at first it fizzes around and starts darting around with considerable speed in the container holding the water. In a few seconds, we will hear a hissing sound with a thick cloud of gas emerging from the container. This gas is actually hydrogen, and is caused by the splitting of water molecules due to the immense reactivity of sodium. The reaction is as follows:
$2Na + 2{H_2}O \to {H_2} + 2NaOH$
Since the reaction is highly exothermic, it releases a lot of heat, and the liberated hydrogen gas catches fire due to this heat. As we know, hydrogen burns with a characteristic pop sound, which confirms that it was the liberated gas.

So, the correct answer is Option C.

Note: The fire is a bright orange flame and usually subsides after a few seconds, due to all the sodium being used up. Since it catches fire even in the presence of very low moisture, sodium metal is always stored by immersing it in kerosene. The high reactivity of sodium can be attributed to the fact that it has just one electron in its valence shell, and thus, it always tries to lose this electron and form a sodium ion ($N{a^ + }$).