Washing soda and iron [III] chloride are separately exposed to the atmosphere. State

(i) the observations seen (ii) the phenomenon which occurs (iii) the reason for the phenomenon occurring in each case.

Would a similar phenomenon occur in case of exposure of common salt. Explain giving reasons.

When Washing soda is exposed to the atmosphere:

(i) Observations — It loses it's moisture [ i.e., water of crystallization] and becomes amorphous.

(ii) Phenomenon — Efflorescence

(iii) Reason for Efflorescence — Efflorescence occurs when the vapour pressure of the hydrated crystals exceeds the vapour pressure of the atmospheric humidity. Due to this, the substance loses its water of crystallization to the atmosphere.

When Iron (III) Chloride is exposed to the atmosphere:

(i) Observations — It absorbs moisture from the atmosphere and changes to liquid state.

(ii) Phenomenon — Deliquescence

(iii) Reason for Deliquescence — Deliquescence occurs when the vapour pressure of the deliquescent crystal solution is very low compared to the vapour pressure of the atmospheric humidity.

Exposure of common salt to the atmosphere:

No, in case of common salt (NaCl) no such phenomenon can occur as, it is anhydrous salt and does not contain water of crystallization.
Common salt is not deliquescent but contains small amount of calcium or magnesium chloride impurities which are deliquescent.
Hence, the impurities absorb moisture from the air, thereby turning common salt sticky and wet.