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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.

Efflorescence

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.

Hygroscopy

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.

Deliquescence

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.

 

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