Question

Differentiate between an efflorescent, deliquescent & ‘ hygroscopic substance with suitable examples.

Answer 1

• Lose wholly or partially their water of crystallisation when exposed to air.
• Become powdery.
• Washing soda CaCO₃.10H₂O, Gauber’s salt

Na₂SO₄.10H₂O, epsom salt MgSO₄.7H₂O 

\[\ce{Na2CO.10H2O->[dry air]Na2CO3.H2O + 9H2O}\]

• Crystalline when hydrated.

Deliquescent substances:

1. Absorb moisture from atmosphere, absorb water and dissolve in it forming a saturated solution.
2. Deliquescence is maximized in dry conditions,
3. NaOH, KOH, CaCl₂, MgCl₂, FeCl₃ are the examples.
4. They are crystalline water soluble.
   FeCl₃ → FeCl₃ [Crystal changes to Solution]

Hygroscopic substances:

1. They are amorphous solids or liquids.
2. Absorb moisture from the air of atmosphere.
3. Do not change their original state.
   CaO (quick lime), cone. H₂SO₄, P₂O₅ and silica gel are the examples.

\[\ce{\underset{\text{Conc.}}{H2SO4} ->\underset{\text{Conc.}}{H2SO4}}\]

(State not changed)

is used for drying gases in laboratory.