When a salt loses its water of crystallization to the atmosphere on exposure, the process is said to be
Efflorescence
Deliquescence
Effervescence
Fluorescence
Explanation
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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, Na2 SO 4. 10H 2O loses all its water of crystallization when exposed to air; Na2 CO 3 . 10H 2 O loses 9 of its molecules of water of crystallization; and FeSO 4 . 7H 2O 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. H 2SO 4 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 , CaCl 2. 6H 2O, P 4 O 10 (forms HPO 3), FeCl3 , KOH, MgCl 2 .
Note: substances which absorb water from air can be used as drying agents for gases. Example, CaCl2 and P 4O 10 can be used to dry gases, but not ammonia. Conc. H 2 SO 4 is used to dry HCl. CaO or silica gel is suitable for drying ammonia gas, this is because CaCl 2 , P 4O 10 and H 2 SO4 will react with the ammonia.
