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I have learnt that deliquescent substances are the ones which absorb moisture from the surroundings and convert them to solution and efflorescent substances are the ones which release moisture into the surroundings.

But I am confused about the mathematical relations used to determine whether the substance will be efflorescent or deliquescent ?

My attempt at trying to understand it :

  • I have read that for deliquescent substances the vapour pressure of the solution formed should be less than the partial pressure of water vapour in air and it should be the other way round for efflorescent substances.

  • But I am unsure about this as my teacher compared such the behaviour of such substances to the real life example that if we want to come out of a door, the pressure exerted by us should be more than the pressure exerted from outside and this makes me think that for deliquescent and efflorescent substances I should rather be comparing vapour pressure of the solution or the substance with the total atmospheric pressure?

I am really confused about this. I tried searching for this elsewhere but couldn’t be sure about this.

Any help would be greatly appreciated!

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    $\begingroup$ Total pressure plays role only if we consider boiling conditions, otherwise partial vapor pressure is considered. Thus applies as well to pure water as to this case. $\endgroup$
    – Poutnik
    Jul 10, 2021 at 7:59
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    $\begingroup$ First forget about atmospheric pressure. The vapor pressure of a substance is typically not significantly affected by the applied pressure, at least under atmospheric conditions. Now consider how you might run an experiment. You take a weighted sample of the dry substance and place it in a sealed container (filled say with an inert gas) together with a drop of water (not in contact). You control the temperature and wait. Some time later you take the sample and again measure its weight. If the weight increased, it is deliquescent, if the weight decreased, it is efflorescent. $\endgroup$
    – Buck Thorn ♦
    Jul 12, 2021 at 17:42

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