Chemistry - Why isn't dry CaCl₂ used to dry HCl gas instead of concentrated sulfuric acid?

Solution 1:

There are several reasons:

1. Efficiency
   When a stream of wet hydrogen chloride is run through a tube filled with coarse calcium chloride "rocks", interaction with the drying agent takes place at at its surface.
   When the same wet $\ce{HCl}$ ist passed through sulfuric acid, the contact area is given by the surface area of the bubbles. Moreover, bubbles mean mixing. The trapped water is distributed over the drying agent.

2. Safety
   Wet calcium chloride will clog, which might result in a pressure buildup in the drying tube.

Solution 2:

Why does concentrated $\ce{H_2SO_4}$ oxidise? Due to the nascent oxygen it gives: $$\ce{H_2SO_4}\rightleftharpoons\ce{SO_2 +H_2O +[O]}$$

Now, if you want $\ce{HCl}$ to be oxidised, the following reaction must exist: $$\ce{HCl} +\ce{[O]}\rightarrow\ce{HClO}$$

But, you can observe that the reaction$$\ce{HClO}\rightarrow\ce{HCl} +\ce{[O]}$$ can't be backward under normal conditions (you probably have read this reaction in the bleaching action of bleaching powder). So, fortunately, you can safely use $\ce{H_2SO_4}$ in this purpose...and take its advantage to $\ce{CaCl_2}$, as agha rehan abbas and Molx mentioned.