Question

Moist hydrogen gas combines with chlorine in the presence of diffused sunlight to form $HCl$. The catalyst used here:
A.Oxygen
B.Silica
C.Alum
D.Moisture

Answer 1

To answer this question, you should recall the concept of $HCl$ gas, its preparation and properties. The air is a mixture of nitrogen, oxygen, carbon dioxide and water vapour along with trace amounts of other gases.

Complete step by step answer:
$HCl$ is an uncoloured gas and has a pungent aroma. Hydrochloric acid is the aqueous solution of hydrogen chloride. $HCl$ is soluble in water. It liquefies $189K$ to form a colourless liquid and freezes at 159k to form a white solid.
The moisture acts as a catalyst in the formation of $HCl$ gas. It accelerates the reaction. It provides a reacting medium to hydrogen and chlorine gas. It increases the interaction between them. The reaction of chlorine with moist hydrogen gas is explosive in case of direct sunlight.
In dark, no reaction will take place because activation light is required. HCl is used in the preparation of chlorine, aqua regia, and other chlorides. It is used as a solvent to dissolve noble gases. It acts as a reagent in laboratories.

Hence, the correct option is D.

Note:
You should note that at room temperature, hydrogen chloride is a colourless to slightly yellow, corrosive, nonflammable gas that is heavier than air and has a strong irritating odour. On exposure to air, hydrogen chloride forms dense white corrosive vapours. Hydrogen chloride can be released from volcanoes. Hydrogen chloride has many uses, including cleaning, pickling, electroplating metals, tanning leather, and refining and producing a wide variety of products. Hydrogen chloride can be formed during the burning of many plastics. Upon contact with water, it forms hydrochloric acid. Both hydrogen chloride and hydrochloric acid are corrosive.