Efflorescence, deliquescent and hygroscopic compounds are terminologies used in chemistry to describe the chemical changes that occur in a compound during a chemical reaction due to its lose or gain of moisture or water of crystallization when exposed to the air in their surroundings.
When pellets of sodium hydroxide, washing soda crystals that is gotten from an airtight container, concentrated tetraoxosulphate(VI) acid and quicklime are each placed on a watch glass after noting their appearances carefully. When you leave each of these substances in the open for a while, you would notice some visible differences in their appearance which is an indication of their ability to either lose or gain moisture from the atmosphere.
Efflorescence: A number of crystalline salts will lose all or part of their water of crystallization when they are exposed to the atmosphere to form a lower hydrate or the anhydrous salt. This phenomenon is referred to as efflorescence while the salt is said to be efflorescent. A perfect example of a compound that undergoes efflorescence is washing soda where its molecules lose nine out of its ten molecules of water of crystallization when it is exposed to the open air.
Na2CO3.10H2O(s) → Na2CO3.H2O(s) + 9H2O
Deliquescence: Deliquescence is a phenomenon whereby a compound absorbs so much amount of water from the atmosphere so that it eventually turns into a solution. Salts that undergo this kind of process are said to be deliquescent. Examples of these kind of salts or compounds are
(i) Iron(II) chloride
(ii) Magnesium chloride
(iii) Potassium hydroxide
(iv) Sodium hydroxide
(v) Phosphorus(V) oxide and
(vi) Calcium chloride
Hygroscopy:– A hygroscopic substance will absorb moisture from the atmosphere but would not form a solution. It would only become sticky if it is a solid. Hygroscopic liquids such as concentrated tetraoxosulphate(VI) acid will absorb water from the atmosphere and would usually dilute itself up to three times of its original volume. Hygroscopic substances are very useful as drying agents in the laboratory. Some examples of other hygroscopic substances are
(i) Sodium trioxonitrate(V)
(ii) Copper(II) oxide and
Drying Agents:– Drying agents are substances or compounds that have very strong affinity for water or moisture. These substances could either be deliquescent or hygroscopic. They are usually used to dry gases in the laboratory and are also commonly used in desiccators. It must be noted that a drying agent cannot be used if it reacts with the substance to be dried. For instance, concentrated tetraoxosulphate(VI) acid cannot be used to dry a compound like ammonia since it would react with it to form ammonium tetraoxosulphate(VI).
2NH3(g) + H2SO4(aq) → (NH4)SO4(aq)