# Conditions for a substance to show deliquescent or efflorescent behaviour

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?