Question

Hydrogen chloride gas is collected by ______?

Answer 1

Out of the different methods available for collecting gases produced in laboratories, the method selected for collecting hydrogen chloride gas is based on its density (whether its heavier or lighter than air) and its solubility in water.

Complete answer: There are various methods of synthesizing a gas through chemical reaction in laboratories. These gases released as part of the product are difficult to control and store. Also it is difficult to get an estimate of the amount of gas produced by a certain specified quantity of reactants.
Hydrogen chloride is an acidic and colourless gas that can be easily identified on the basis of its unique odor. The high solubility of hydrogen chloride in water makes it difficult to collect the gas over water (using water displacement technique). The gas is produced by a reaction between sodium chloride and concentrated sulphuric acid and collected by a technique called downward delivery (upward displacement of air). The gas being $1.28$ times heavier than air, is best collected through this method.
$NaCl + conc.{H_2}S{O_4} \to HCl \uparrow (fumes) + N{a_2}S{O_4}$
As the name suggests, a jar is fixed in such a position so as to receive the evolved gas in the downward direction, the air present inside the jar keeps getting displaced.
Thus hydrogen chloride gas is collected by downward delivery (upward displacement of air) technique.

Additional information: The most common method of collecting the gases produced through chemical reactions is the water displacement technique. This method utilizes an inverted vessel filled with water and the produced gas is bubbled into it. The gas is obtained as a mixture of the gas and water vapour. Dalton's law of partial pressures is useful in determining the partial pressure exerted by the gas alone by subtracting the aqueous tension (partial pressure of water vapour) from the total pressure exerted by the mixture. Though the technique is easy to use and economically suitable, yet it cannot be used for the gases that themselves get dissolved in water.

Note:
The reason for using sodium chloride salt to produce hydrogen chloride gas instead of any other chloride salt is because it is cheap and abundantly available. The concentrated sulphuric acid cannot be replaced by nitric acid as it has a tendency to volatilize along with hydrogen chloride gas.