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So our teacher asked us a simple question in the classroom:

What happens when Ar and HCl are mixed at 77 K?

and no other information was given.

We replied various possible answers for that such as Debye bond will form between Ar−HCl, force of attraction is proportional to the inverse sixth power of Ar−HCl intermolecular distance, Ar has its octet filled, it separates into a different layer.

Though our teacher said all of them are wrong, but I am facing some trouble understanding how the other students get there. Could anyone help me with some hints?

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    $\begingroup$ What do you suppose the boiling points and melting points of HCl and Argon are? $\endgroup$ – Waylander Nov 12 '19 at 8:48
  • $\begingroup$ Well, we asked if we could take 159k as the melting point, but he said that we don't need to go for the melting or boiling point $\endgroup$ – kiv Nov 12 '19 at 8:54
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    $\begingroup$ pubs.acs.org/doi/10.1021/j100382a027 $\endgroup$ – Nilay Ghosh Nov 12 '19 at 8:56
  • $\begingroup$ @NilayGhosh The article isn't free, could you help me with something else $\endgroup$ – kiv Nov 12 '19 at 9:54
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    $\begingroup$ What’s the pressure? $\endgroup$ – Chet Miller Nov 12 '19 at 14:31
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At 77 K, both HCl and Ar are solids. So if you cool those gases separately and mix the two solids, you obtain a simple mixture of two powders.

If you mix Ar and HCl at room temperature, you obtain a mixture of gases. By cooling this mixture slowly, HCl will first liquefy at -83°C (density 1.194), then solidify at -112°C. Later Argon will liquefy at -185°C in a liquid heavier than HCL crystallized, with density 1.4, then solidify at -189°C in a solid (density 1.65). You will get two solid layers : HCl above Ar. But there is no chemical reaction between HCl and Ar

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    $\begingroup$ What if the pressure is very low so that they are each below their equilibrium vapor pressures? $\endgroup$ – Chet Miller Nov 12 '19 at 18:21

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