In our chemistry books, it is said that concentrated sulphuric acid is used to dry $\ce{HCl}$ gas.

But, concentrated sulphuric acid is a strong oxidising agent so, why isn't $\ce{HCl}$ gas oxidised to $\ce{H2O}$ and chlorine gas?

Why dry $\ce{CaCl2}$ (Calcium Chloride) isn't used as drying agent?

[Note: $\ce{CaCl2}$ have no reaction with $\ce{HCl}$ as they both have chloride ion in common]

  • $\begingroup$ cacl2 is a solid (amorphous mostly) and it is hard to handle with amorphous powdery solid, because they get mixed with the solution to be dried $\endgroup$ Commented Apr 27, 2015 at 10:51
  • 1
    $\begingroup$ Does the book mention more details about the process? Specifically, is it talking about drying the $\ce{HCl}$ gas, or producing it from $\ce{HCl}$ solution? For the $\ce{CaCl2}$ part, I'd start with the physical state: bubbling a gas through a liquid seems a lot more efficient than through a solid. $\endgroup$
    – Molx
    Commented Apr 27, 2015 at 13:48
  • $\begingroup$ Sulphuric acid isn't oxidizing enough to do this. $\endgroup$
    – Mithoron
    Commented May 2, 2015 at 23:26

3 Answers 3


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.


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.

  • $\begingroup$ Oxygen is more electronegative than chlorine so H2O is more stable compound than HCl .Hence,it is very possible to form H2O by oxidising HCl $\endgroup$
    – Swastik
    Commented Aug 6, 2015 at 11:45

This is because calcium is higher than hydrogen in the activity series of metals due to which, the objective of the reaction will be destroyed. This is why concentrated sulphuric acid is used.

  • $\begingroup$ Can you elaborate your bit more? $\endgroup$
    – Freddy
    Commented Apr 29, 2015 at 9:36
  • 7
    $\begingroup$ Calcium metal is higher in the activity series; calcium chloride and hydrogen chloride do not react at all, as mentioned at the end of the question. $\endgroup$ Commented Apr 29, 2015 at 11:06

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