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Efflorescence, Hygroscopy, and Deliquescence

Efflorescence, Hygroscopy, and Deliquescence are concepts which explain the changes that occur when some substances are exposed to air.

When some substances are exposed to air, they lose water to the atmosphere, thereby reducing in weight. Solids that behave in this way are those with water of crystallization. The molecules of water of crystallization are partially or completely lost to the atmosphere, thereby making them lose their crystalline forms.

Example, Na₂SO₄. 10H₂O loses all its water of crystallization when exposed to air; Na₂CO₃. 10H₂O loses 9 of its molecules of water of crystallization; and FeSO₄. 7H₂O loses all its molecules of water of crystallization.

This is when substances absorb water from air, but not enough to form solutions. Examples of such substances include CaO, NaNO₃, NaCl, Sucrose and CuO. Also, certain liquid substances absorb water from the air to get diluted - these are also regarded as being hygroscopic. Example, conc. H₂SO₄ and conc. HCl. lf a hydroscopic substance absorbs so much moisture that an aqueous solution is formed, the substance becomes deliquescent.

This is when certain solid substances, when exposed to air absorb water, enough to form solutions. Example, solid NaOH, CaCl₂, CaCl₂. 6H₂O, P₄O₁₀ (forms HPO₃), FeCl₃, KOH, MgCl₂.

Note: substances which absorb water from air can be used as drying agents for gases. Example, CaCl₂ and P₄O₁₀ can be used to dry gases, but not ammonia. Conc. H₂SO₄ is used to dry HCl. CaO or silica gel is suitable for drying ammonia gas, this is because CaCl₂ , P₄O₁₀ and H₂SO₄ will react with the ammonia.