Question

Define deliquescence and efflorescence with suitable examples.

Answer 1

Deliquescence: Certain substances which are water-soluble, when exposed to the atmosphere at ordinary temperature, absorb moisture and becomes moist, and finally dissolve in the absorbed water, forming saturated solutions are called deliquescent substance and the phenomena are called deliquescence.

• This happens when the vapor pressure inside the deliquescent substance is very low as compared to the outside atmosphere.
• The phenomenon is slow in humid conditions.
• Example: Table salt is a deliquescent substance, it first absorbs moisture and then dissolved in it forming a saturated solution. Similarly, Caustic soda $\left(\mathrm{NaOH}\right),$ caustic potash $\left(\mathrm{KOH}\right)$, Magnesium chloride $\left({\mathrm{MgCl}}_2\right)$, etc. are all deliquescent substances.

Efflorescence: It is a phenomenon where a substance loses its water of crystallization on exposure to dry air.

• It happens when the vapoUr pressure of the crystal is high in comparison to the pressure of the outer atmosphere.
• The phenomenon is slow in humid conditions.
• Example: Washing soda when exposed to dry air for a shorter duration loses its water of crystallization.

$\underset{(\mathrm{Washing}\mathrm{soda})}{{\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}\stackrel{\mathrm{dry}\mathrm{air}}{\to }\underset{(\mathrm{Sodium}\mathrm{carbonate}\mathrm{monohydrate})}{{\mathrm{Na}}_2{\mathrm{CO}}_3.{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{9{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$.