Efflorescent, deliquescent and hygroscopic compounds are terminologies used in chemistry to describe the chemical changes that occur in a compound during a chemical reaction. These changes are reflective of the kind of bonding present in the reacting elements, their oxidation numbers, their affinity for electrons and their ability to displace other elements.
Certain hydrated crystalline salts when exposed to the atmosphere at ordinary temperature lose their water of crystallisation molecules either partially or completely and become anhydrous. This process occurs when the aqueous vapour pressure of the hydrate is greater compared to the partial pressure of the water vapour in the air.
We can say Efflorescence, the automatic loss of water by aqueous salts, which occurs when the pressure of the hydrate vapour is greater than the partial pressure of the air vapour. For example, Na2SO4.10H2O, FeSO4.7H2O, Blue Vitriol (CuSO4.5H2O), washing soda (Na2CO3.10H2O) lose 9 water molecules first and then become completely anhydrous.
Deliquescence is a phenomenon whereby a compound absorbs so much amount of water from the atmosphere so that it eventually turns into a solution. Salts that undergo this kind of process are said to be deliquescent. Examples of these kind of salts or compounds are
(i) Iron(III) chloride
(ii) Magnesium chloride
(iii) Potassium hydroxide
(iv) Sodium hydroxide
A hygroscopic substance will absorb moisture from the atmosphere but would not form a solution. It would only become sticky if it is a solid. Hygroscopic liquids such as concentrated tetraoxosulphate(VI) acid will absorb water from the atmosphere and would usually dilute itself up to three times of its original volume. Hygroscopic substances are very useful as drying agents in the laboratory. Some examples of other hygroscopic substances are
(i) Sodium trioxonitrate(V)
(ii) Copper(II) oxide and
Also, certain liquid substances absorb water from the air to get diluted, these are also regarded as being hygroscopic. Example, conc. H2SO4 and conc. HCl. lf a hydroscopic substance absorbs so much moisture that an aqueous solution is formed, the substance becomes deliquescent.