Deliquescence

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Define Deliquescence

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.


Efflorescence

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. 


Hygroscopy

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.


Difference Between Efflorescent Substances, Deliquescent Substances, and Hygroscopic Substances

Efflorescent Substances

Deliquescent Substances

Hygroscopic Substances

They lose their water of crystallization either partially or wholly when exposed to air.

They absorb water and moisture from the atmosphere and dissolve in it.

They are amorphous solids or liquids.

They become powdery.

The effect of the substances is maximum in dry conditions.

They absorb moisture from the air present in the atmosphere.

They get crystalline when hydrated.

They are crystalline water-soluble substances.

The original state is not changed by them.

Eg: Blue Vitriol (CuSO4.5H2O) and washing soda (Na2CO3.10H2O).

Eg:  NaOH, KOH, MgCl2, CaCl2, FeCl3, LiCl, etc.

Eg: Zinc chloride, sodium chloride, sodium hydroxide crystals, etc.


Did You Know?

In the rainy season, you might have noticed that the salt turns sticky, This is due to the reason that NaCl contains small impurities of MgCl2, CaCl2 which makes the salt sticky. The effectiveness of calcium chloride in settling road dust is also an example of its deliquescence. When it is spread in the form of a powder or flakes, it can absorb more water than its weight and forms a liquid that can keep the road wet.


FAQ (Frequently Asked Questions)

1. Define deliquescence and give some examples of deliquescence substances.

According to the Deliquescence definition 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 tendency to become liquid.


Examples are:

Hydroxides:  NaOH, KOH

Chlorides:  MgCl2, CaCl2, FeCl3, LiCl

Nitrates:  Cu(NO3)2, NaNO3,LiNO3

2. What is the main difference between Hygroscopy and Deliquescence?

Hygroscopic and deliquescent materials both can absorb moisture from the air. But, hygroscopy and deliquescence are not the same things: Hygroscopic materials absorb moisture On the other hand deliquescent materials absorb moisture to the extent that the substance dissolves in water.

3. What is the difference between deliquescent and Efflorescence?

Deliquescence refers to a process in which a highly hygroscopic substance absorbs water from the atmosphere to such an extent that the substance dissolves in it. On the other hand, efflorescence refers to a process in which salt present within a porous material is first dissolved by water and then transferred to the surface of the porous material where the salt precipitates out of solution.