Question

Define the terms efflorescence and deliquescence.

Answer 1

Hint: These two terms are related to surface chemistry. It is a property of some substances.

Complete step by step answer:
Let us look the definitions of each terms, “Efflorescence refers to the action of movement of salty liquids to the surface of a porous material, where the liquid evaporates leaving a white powdery coating on the surface” and “Deliquescence is the process by which a chemical substance absorbs moisture from the atmosphere and dissolves in it”.
Efflorescence leads to the powdery white coat normally seen on brick and rocks surfaces. Deliquescence occurs when the vapour pressure of the formed solution is less than the partial pressure of water vapour in the atmosphere. All soluble salts will deliquesce if the air is sufficiently humid.

Additional Information:
Hygroscopic substances: This is when substances absorb water from air, but not enough to form solutions. Examples of such substances include CaO, NaCl, sucrose and CuO. Also, certain liquid substances absorb water from the air to get diluted, these are also regarded as being hygroscopic. Examples are concentrated ${\text{H}}_{\text{2}}\text{S}{\text{O}}_{\text{4}}$ and concentrated HCl. If a hygroscopic substance absorbs so much moisture that an aqueous solution is formed, the substance becomes deliquescent.
Examples for efflorescent substances are $\text{N}{\text{a}}_{\text{2}}\text{S}{\text{O}}_{\text{4}}\text{.10}{\text{H}}_{\text{2}}\text{O}$, $\text{N}{\text{a}}_{\text{2}}\text{C}{\text{O}}_{\text{3}}\text{.10}{\text{H}}_{\text{2}}\text{O}$ and $\text{FeS}{\text{O}}_{\text{4}}\text{.7}{\text{H}}_{\text{2}}\text{O}$.
Examples for deliquescent substances are NaOH, $\text{CaC}{\text{l}}_{\text{2}}$, $\text{CaC}{\text{l}}_{\text{2}}\text{.6}{\text{H}}_{\text{2}}\text{O}$, etc.

Note: Most of the time students get confused with these two words. Remember that deliquescence absorbs moisture while efflorescence is loss of water by a hydrated salt.