(a) The gases hydrogen and chlorine don't react with each other, even if kept together for a long time. However, in the presence of sunlight, they readily combine. What actually happens?
(b) A water-insoluble substance 'X' on reacting with dilute H₂SO₄ released a colorless and odorless gas accompanied by brisk effervescence. $({CO}_{2})$ When the gas was passed through water, the solution obtained turned blue litmus red. On bubbling the gas through lime water, it initially became milky and the milkiness disappeared when the gas was passed in excess. Identify the substance 'X' and write the chemical equations of the reactions involved.

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Solution

(a)
We know that in chemical reactions, energy is needed to break the bonds present in the reacting molecules so that they may combine to form the products.
In the present case, sunlight is the source of energy in the form of photons. The energy made available helps in breaking the bonds present in the reactant molecules and the chemical reaction leading to hydrogen chloride gas takes place.
The chemical reaction in presence of sunlight.
$H_{2} (g) + Cl (g) \overset{sunlight}{\to} 2 HCl (g)$
(b)
When water-insoluble substance ‘X’ reacts with sulphuric acid, gives most probably metal carbonate because acid reacts with metal carbonate and released a colorless and odorless gas accompanied by brisk effervescence. So, therefore ‘X’ is most probably the calcium carbonate $({CaCO}_{3})$ ${CaCO}_{3} (s) + H_{2} {SO}_{4} (dil .) \to {CaSO}_{4} (s) + H_{2} O (aq) + {CO}_{2} (g)$
When the gas $({CO}_{2})$ was passed through water, the solution obtained turned blue litmus red.
${CO}_{2} (g) + H_{2} O (aq) \underset{}{\to} \underset{(turns blue litmus red)}{H_{2} {CO}_{3} (aq)}$
On bubbling the gas through lime water, it initially became milky and calcium carbonate is formed $({CaCO}_{3})$
$Ca {(OH)}_{2} (aq) + {CO}_{2} (g) \to {CaCO}_{3} (s) + H_{2} O (l) (lime water) (milky)$
The milkiness disappeared when the gas was passed in excess and calcium bicarbonate is formed $Ca ({HCO}_{3})$ .
${CaCO}_{3} (s) + CO (g) + H_{2} O (aq) \to Ca {({HCO}_{3})}_{2} (aq)$

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