Deliquescent, Hygroscopic and Efflorescent Compounds

Deliquescent, Hygroscopic and Efflorescent Compounds

These are substances composed of two or more kinds of atoms (elements) joined together in a definite grouping.

The properties of a compound are distinct and different from those of the individual elements that are combined in its make-up.

There are several millions of compounds known. These compounds are derived from the elements e.g. water consists of hydrogen and oxygen atoms in ratio 2 to 1; common salt consists of sodium and chlorine atoms in ratio 1 to 1.

Compounds are usually classified in a number of ways:

Based on the behavior when exposed to the atmosphere, compounds are classified into three: deliquescent, hygroscopic and efflorescent.

Deliquescent compounds are those compounds that absorb moisture from the atmosphere and form the solutions of tjr compounds e.g. sodium hydroxide pellets, anhydrous calcium chloride and iron(III)chloride.

Hygroscopic compounds are those that absorb moisture from the atmosphere and become wet. Their ability to absorb water from air is less than that of deliquescent compounds. Hence the solid ones from pasty substances and not solution while the liquid ones become diluted e.g. sodium trioxonitrate(V), copper(II)oxide and concentrated tetraoxosulphate(VI)acid.

Efflorescent compounds are compounds that give out their water of crystallization to the atmosphere at ordinary temperatures e.g. sodium trioxocarbonate(IV)decahydrate loses nine of its ten moles of water of crystallization on exposure to air.

It should be noted that all deliquescent substances are also hygroscopic but not all hygroscopic substances are deliquescent. Hygroscopy is the ability of a substance to draw water vapour to itself and become wet. Both hygroscopic and deliquescent substances have this ability.

However, if a substance goes beyond being wet and becomes a solution in the course of drawing water to itself, it shows deliquescency.

Deliquescent and hygroscopic compounds are used as drying agents for gases in the laboratory. For instance, concentrated tetra of soul phage(VI)acid is the usual drying agent for acidic gases e.g. sulphur(IV)oxide, calcium oxide is commonly used as the drying agent for the alkaline gas, ammonia. Also, desiccants in desiccators are deliquescent or hygroscopic substances e.g. silica gel, calcium oxide, fused calcium chloride and phosphorus(V)oxide.