Question

Briefly discuss the following with examples: a. Efflorescent substance
b. Deliquiscent substance
c. Hygroscopic substance

Answer 1

Brief definitions, mechanism and examples:

a) Efflorescent substance
- Definition: A crystalline hydrate that loses its water of crystallization on exposure to air, becoming powdery or anhydrous.
- Mechanism: The vapor pressure of the water in the hydrate is higher than the ambient water vapor pressure, so water evaporates from the crystal.
- Example: Copper(II) sulfate pentahydrate (CuSO4·5H2O) when exposed to air can lose water and turn from blue (hydrated) to white (anhydrous CuSO4): CuSO4·5H2O → CuSO4 + 5 H2O (g).

b) Deliquescent substance
- Definition: A solid that absorbs so much moisture from the air that it dissolves in the absorbed water and forms an aqueous solution.
- Mechanism: The salt’s affinity for water and low equilibrium vapor pressure cause continuous uptake of water vapor until the solid liquefies. The critical relative humidity at which this happens is called the deliquescence relative humidity (DRH).
- Examples: Calcium chloride (CaCl2), sodium hydroxide (NaOH) and magnesium chloride (MgCl2) — pellets become wet and dissolve on exposure to humid air.

c) Hygroscopic substance
- Definition: A substance that absorbs moisture from the air but does not necessarily dissolve; it may just become damp or swell.
- Mechanism: Strong attraction for water vapor (but uptake may be limited so the solid remains solid).
- Examples: Silica gel (used as a desiccant), concentrated sulfuric acid (very hygroscopic), glycerol. (Note: deliquescent materials are a subset of hygroscopic materials whose uptake is large enough to produce a liquid.)

Practical note: store hygroscopic/deliquescent reagents in tightly closed containers or desiccators; efflorescent hydrates are often handled in closed containers to prevent loss of water of hydration.