Question

When calcium metal is added to water the gas evolved does not catch fire but the same gas evolved on adding sodium metal to water catches fire. Why is it so?

Answer 1

We need to know the properties of calcium and sodium and how they behave when allowed to react with water. Calcium is an atomic number $20$ chemical element with the symbol$Ca$. Calcium is a reactive alkaline earth metal that forms a dark oxide-nitride coat when exposed to oxygen. The chemical element sodium has the symbol $\,Na$ and the atomic number $11$. It's a soft, silvery-white metal with a high reactivity.
Sodium belongs to the periodic table's group $1$ as an alkali metal.

Complete answer:
When calcium metal is added to water the gas evolved does not catch fire but the same gas evolved on adding sodium metal to water catches fire. ${H_2}$ is the gas that is generated in both situations. When calcium reacts with water the heat evolved is not sufficient for hydrogen to catch fire. Calcium hydroxide, $Ca{\left( {OH} \right)_2}$, and hydrogen gas are produced in this reaction (${H_2}$). The calcium metal falls in water, and after about an hour or so, bubbles of hydrogen appear on the metal's surface. On the other side, sodium metal reacts violently with water, releasing a large amount of heat, which is necessary for hydrogen to catch fire. The rapid reaction of sodium metal with water produces a colourless solution of sodium hydroxide ($NaOH$) and hydrogen gas (${H_2}$). Because of the dissolved hydroxide, the resulting solution is basic. It's an exothermic reaction.

Note:
We must note that a reaction in which energy is emitted in the form of light or heat is known as an exothermic reaction. As a result, rather than extracting energy from the environment as in an endothermic reaction, energy is passed onto the surroundings in an exothermic reaction. The difference in enthalpy ($H$) in an exothermic reaction is negative.