Question

What does the pop sound indicate when sodium reacts with cold water?
a.) Release of oxygen
b.) Release of carbon dioxide
c.) Both A and B
d.) Release of hydrogen

Answer 1

Hint: Sodium metal is very reactive. Sodium reacts vigorously with oxygen and water. A large amount of heat is generated in the reaction. That’s why we store sodium metal in kerosene.

Complete step by step solution: Metals react with water and produce a metal oxide and hydrogen gas
${ Metal+water\rightarrow Metal\ oxide +{ H }_{ 2 } }$
However, if the metal oxide is soluble in water it further dissolves in water to form the corresponding metal hydroxide.
${ Metal\quad oxide+water\rightarrow Metal\ hydroxide }$
Hence, for such metals, whose oxides are soluble in water, the overall reaction of the metal with water can be written as:
${ Metal+water\rightarrow Metal\ hydroxide+H_{ 2 } }$

Sodium in reaction with water forms sodium oxide, which is quite soluble in water. This further dissociates in water to form sodium hydroxide, because of which the overall reaction with water is written as:
${ 2Na(s)+{ 2H }_{ 2 } }{ O(l)\rightarrow 2NaOH+{ H }_{ 2 } }$
To test for the release of hydrogen gas, a lighted splint is placed over the reacting metal- if it “pops” then hydrogen gas is present.
So, the correct option is D. When sodium metal reacts with cold water then hydrogen gas is released.

Note: The possibility to make a mistake is that you may choose option (b). But carbon dioxide is not released when sodium metal reacts with cold water.