Efflorescent, Deliquescent, and Hygroscopic Substances

1.      In efflorescence, the vapor pressure of the hydrated salt is greater than the vapor pressure of water vapor in the atmosphere. The salt tries to equalize pressure but, in view of the size of the atmosphere, this can never be achieved, and so it passed to a lower hydrate or the anhydrous salt, with a lower vapor pressure.

Na₂CO₃ · 10H₂O → Na₂CO₃ · H₂O + 9H₂O

2.      In deliquescence, the vapor of the salt hydrate (and its saturated solution) is less than the vapor pressure of water vapor in the atmosphere. The salt tries to equalize pressures by absorbing water from the atmosphere, forming eventually an unsaturated solution.

MgCl₂, 6 H₂O, P₂O₅, NaOH

It should be remembered that the water vapor present in the atmosphere is not constant, varying from day to day and especially from summer to winter. Hence, there is a possibility that certain compounds may be either efflorescent or deliquescent depending on the season or the locality.
Ordinary common salt is not deliquescent but because of MgCl₂ which is an impurity, it dissolves and forms a solution in rainy reason and becomes deliquescent.

Calcium chloride is sprinkled on the road to remove dust as it absorbs moisture and forms a saturated solution by dissolving in it. It is used as a dehydrating agent. The term deliquescent is used for solids only.

3.      A hygroscopic substance (e.g. conc. H₂SO₄, CuO) absorbs water from the atmosphere without forming a solution.
Human hair is hygroscopic.
H₂SO₄ is not deliquescent but it is hygroscopic and used as a dehydrating agent.

Drying Apparatus

To Dry	Drying Agent	Apparatus
Gas	·            Conc. H2SO4 ·        P2O5 ·        CaCl2	Woulfe’s bottle
Gas	·        P2O5 ·            CaCl2 ·        CaO	Drying Tower
Gas	·        (Salt and ice) Freezing mixture	U tube
Liquid	·        Conc. H2SO4	Bottle
Solid	·        Silica gel ·        CaO ·        Anhydrous CaCl2	Dessicator
Solid	·        Conc. H2SO4	Bottle