Certain substances have the property to absorb moisture when exposed to the atmosphere at ordinary temperature, they initially become wet, lose their crystalline form and finally dissolve in water to form a saturated solution. Deliquescent meaning a tendency to become liquid. This process occurs when the vapour pressure of the solution that is formed is less compared to the partial pressure of water vapour in the air. Deliquescence substances are solids that tend to absorb moisture from the air and dissolve it. Eg: NaOH, KOH, MgCl2, CaCl2, FeCl3, LiCl, Cu(NO3)2, NaNO3, LiNO3 etc are deliquescent substances. In this topic, we have covered the deliquescent definition, let’s discuss some other properties like efflorescence and hygroscopic.
Certain hydrated crystalline salts when exposed to the atmosphere at ordinary temperature lose their water of crystallization molecule either partially or completely and become anhydrous. This process occurs when the aqueous vapour pressure of the hydrate is greater compared to the partial pressure of the water vapour in the air. Eg: Blue Vitriol (CuSO4.5H2O), washing soda (Na2CO3.10H2O) loses 9 water molecules first and then becomes completely anhydrous.
Certain substance absorbs moisture from the atmosphere at ordinary temperature but does not dissolve in it. These are called hygroscopic substances and the property is known as hygroscopy. These substances are generally used as a drying agent. When water vapour is absorbed, the water molecules are taken by the molecules of the hygroscopic substance, which results in physical changes like an increase in volume, boiling point, temperature, and changed viscosity. Eg: Zinc chloride, sodium chloride, sodium hydroxide crystals, silica gel, honey, nylon, and ethanol are hygroscopic in nature.
Sulfuric acid is also hygroscopic in nature, not only when it is concentrated but also when reduced down to a concentration of 20% v/v or even lower than that. Germinating seeds are also examples of hygroscopic. Once the seeds are dried, their outer coating becomes hygroscopic and it begins to absorb the moisture required for germination.