Deliquescence: We all must have seen tiny silica gel packets inside shoe boxes, new bags, and other gadgets, and they are there for a reason. Silica is a drying agent also known as a desiccant. It absorbs moisture from its surroundings and keeps objects from moisture. Though the instructions on these packets are to throw them away immediately, these prove to be very useful at home. Just be careful to keep it out of reach of children. In this article, let’s understand about deliquescence in this article.

What is Deliquescence?

The process by which a solid substance turns into liquid due to the absorption of moisture from the atmosphere is known as deliquescence. Water vapour absorption takes place until the solid dissolves in the absorbed water and forms a solution.

How does it Work?

Solids always contain impurities, which lower their melting point.
The absorbed water causes a decrease in the melting point of the solid. If enough water is absorbed to lower the melting point of the solid below its room temperature,  it will deliquesce or turn to liquid. This process occurs when the vapour pressure of the resulting solution is less than the partial pressure of water vapour in the air. If the air is sufficiently humid, all soluble salts will deliquesce.

However, some substances do absorb moisture from the air but not necessarily to the point of dissolution of the solid. These substances are hygroscopic in nature.

Examples of Deliquescent Substances

Deliquescence refers to the property of a substance by virtue of which the substance absorbs moisture from the surrounding and dissolves in it to form an aqueous solution. Materials that exhibit this property are known as deliquescent substances. Deliquescent materials belong to the class of hygroscopic substances. All hygroscopic materials may absorb water, but for a substance to be deliquescent, it must absorb a large amount of water to be sufficiently soluble in it.

Most deliquescent substances are salts, for example, potassium hydroxide, sodium hydroxide, ammonium chloride, sodium nitrate, gold(III) chloride, and calcium chloride. Sodium chloride \(\left( {{\text{NaCl}}} \right)\) or table salt may be deliquescent if the particles are small and have very high humidity. It is usually considered to be hygroscopic.

Applications of Deliquescent Substances

1. Deliquescent substances, due to their affinity for water, are used as desiccants. These find use in removing excess water from chemicals such as sulfuric and phosphoric acids.
2. Packets containing lumps of silica gel are often found in new shoes and bottles. Silica gel is a desiccant that absorbs moisture and keeps the item dry.
3. Desiccants are commonly used in food packaging to retain crispness. Industrially, desiccants are widely used to control the level of water in gas streams.
4. These are also used in the manufacture of insulated windows to prevent moisture condensation between the panes.
5. Desiccants are used as a dryer component of air conditioning systems to help maintain the efficacy of the refrigerant.
6. Bagged desiccants are commonly used to protect cocoa, coffee, various nuts and grains, and other foods, particularly those susceptible to mould and rot when exposed to condensation and humidity.
7. Small packets of desiccants are used in pharmaceutical packaging to keep the atmosphere inside the package below critical levels of water vapour.
8. Desiccants are also used to remove water from solvents in chemical reactions that do not tolerate water, e.g., the Grignard reaction.
9. Desiccants are used to dry electronics damaged by accidental spillage.
10. These can be used with homemade cleaning products to prevent clumping.
11. Towels and gym clothes start to smell in a gym bag. Keeping a few desiccants in the gym bag or clothes and blankets prevents mould, mildew, and odours.
12. Silver jewellery or silverware can be stored with desiccants to slow tarnishing.
13. A desiccant can be stored in a toolbox to soak up extra moisture in the air and prevent rusting.
14. Photos can be stored with a desiccant to protect them from humidity.
15. Windshields get dirty when moisture in the air picks up dust and settles on the inside of the windshield. Storing some desiccants along the dashboard can prevent this.

Efflorescence

The opposite of deliquescence is efflorescence.The process by which spontaneous loss of water takes place by a hydrated salt is known as efflorescence. It occurs when the aqueous vapour pressure of the hydrated salt exceeds the partial pressure of the water vapour present in the atmosphere. For example, washing soda \(\left( {{\text{N}}{{\text{a}}_2}{\text{C}}{{\text{O}}_3}.10\,{{\text{H}}_2}{\text{O}}} \right)\) and Glauber’s salt \(\left( {{\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4}.10\,{{\text{H}}_2}{\text{O}}} \right)\) have a powdery appearance. This is because their aqueous vapour pressure is higher than that of the water vapour in the atmosphere; hence these salts effloresce (i.e., lose all or part of their water of hydration), and their surfaces assume a powdery appearance. However, hydrated cupric sulfate, or blue vitriol \(\left( {{\text{CuS}}{{\text{O}}_4}.5\,{{\text{H}}_2}{\text{O}}} \right),\) has its aqueous vapour pressure lower than the atmospheric vapour pressure. Hence, it undergoes efflorescence only if the air in contact with it is relatively dry.

Difference between Efflorescence and Deliquescence

Efflorescence	Deliquescence
The process by which spontaneous loss of water takes place by a hydrated salt is known as efflorescence.	The process by which a solid substance turns into liquid as a result of absorption of moisture from the atmosphere is known as deliquescence.
It occurs when the aqueous vapour pressure of the hydrated salt is greater than the partial pressure of the water vapour present in the atmosphere.	This process occurs when the vapour pressure of the resulting solution is less than the partial pressure of water vapour in the air.
These substances do not absorb water.	These substances can absorb a high amount of water vapour to form an aqueous solution.
These substances have no considerable affinity for water.	These substances have a very high affinity for water

Hygroscopy

We all must have seen table salt become watery in rainy seasons. Is table salt deliquescent? Let’s try to answer it. Certain substances absorb moisture from the atmosphere at ordinary temperatures, but the moisture absorbed is not enough for the substance to dissolve in it. Such substances are known as hygroscopic substances, and the property is known as hygroscopy. These substances are widely used as drying agents. The hygroscopic substance on absorbing water vapour results in physical changes like an increase in volume, temperature, boiling point, and viscosity. Hence, table salt or sodium chloride does not absorb enough water to form an aqueous solution; therefore, it is a hygroscopic substance. Some other examples are- Zinc chloride, sodium hydroxide crystals, honey, nylon, and ethanol.

The king of chemicals, Sulphuric acid, is also hygroscopic in nature, when it is concentrated and when its concentration is reduced down to \(20\% \,{\text{v}}/{\text{v}}\) or even lower than that. Germinating seeds are also hygroscopic as their outer coating begins to absorb the moisture required for germination.

Hygroscopy and Deliquescence

Deliquescent and hygroscopic materials are both able to absorb moisture from the air. However, hygroscopy and deliquescence do not mean the same. Hygroscopic materials absorb moisture but not to the extent that they will make an aqueous solution, while deliquescent materials absorb moisture to the extent that the substance dissolves in water.

A hygroscopic material absorbing moisture will become damp or caky, while a deliquescent material will liquefy. Deliquescence may be considered an extreme form of hygroscopy.

Hygroscopic materials, due to their ability to attract and hold moisture, are referred to as humectants. However, deliquescent materials are known as desiccants. Humectants used in food, cosmetics, and drugs are salt, sugar, honey, ethanol etc.

All deliquescent materials are hygroscopic but all hygroscopic substances are not deliquescent.

Difference Between Deliquescence and Hygroscopy

Deliquescence	Hygroscopy
The process by which a solid substance turns into liquid as a result of absorption of moisture from the atmosphere is known as deliquescence.	The process by which a solid substance turns caky as a  result of absorption of moisture from the atmosphere is known as hygroscopy.
These substances can absorb a high amount of water vapour to form an aqueous solution.	These substances absorb a water vapour but do not form an aqueous solution.
Deliquescent materials are known as desiccants	Hygroscopic materials, due to their ability to attract and hold moisture, are referred to as humectants.

Summary

Who hasn’t accidentally dropped a phone in the sink or the toilet? We must have tried to dry the parts with a hairdryer, but the heat from a hairdryer can damage your electronics. Thanks to desiccants that can be used to dry out electronics without damaging the equipment.  A desiccant exhibits the deliquescence-the property of a substance to absorb enough water vapour to be dissolved in it. However, certain substances such as the table absorb water vapour but not enough to be dissolved in it. Such substances are hygroscopic.  In this article, we learned the property of deliquescence, efflorescence and hygroscopy. We also learned the basic differences behind these phenomena and their various applications.

Frequently Asked Questions

Q.1. What is deliquescence and efflorescence?
Ans: The process by which a solid substance turns into liquid as a result of absorption of moisture from the atmosphere is known as deliquescence. The opposite of deliquescence is efflorescence. The process by which spontaneous loss of water takes place by a hydrated salt is known as efflorescence.

Q.2. What are deliquescent materials?
Ans: Deliquescence refers to the property of a substance by virtue of which the substance absorbs moisture from the surrounding and dissolves in it to form an aqueous solution. Materials that exhibit this property are known as deliquescent substances. Deliquescent materials belong to the class of hygroscopic substances. All hygroscopic materials may absorb water, but for a substance to be deliquescent, it must absorb a large amount of water to be sufficiently soluble in it.

Q.3. What is the difference between deliquescence and hygroscopic?
Ans:

Deliquescence	Hygroscopy
The process by which a solid substance turns into liquid as a result of absorption of moisture from the atmosphere is known as deliquescence.	The process by which a solid substance turns caky as a  result of absorption of moisture from the atmosphere is known as hygroscopy.
These substances can absorb a high amount of water vapour to form an aqueous solution.	These substances absorb a water vapour but do not form an aqueous solution.
Deliquescent materials are known as desiccants	Hygroscopic materials, due to their ability to attract and hold moisture, are referred to as humectants.

Q.4. Why does deliquescence occur?
Ans: The absorbed water causes a decrease in the melting point of the solid. If enough water is absorbed to lower the melting point of the solid below its room temperature, it will deliquesce or turn to liquid. This process occurs when the vapour pressure of the resulting solution is less than the partial pressure of water vapour in the air. If the air is sufficiently humid, all soluble salts will deliquesce

Q.5. Why is efflorescence a problem?
Ans: Efflorescence isn’t dangerous; it is mostly an esthetic issue. It can lead to potential moisture problems that can cause structural damage to building materials.

We hope this detailed article on Deliquescence helped you in your studies. If you have any doubts, queries or suggestions regarding this article, feel to ask us in the comment section and we will be more than happy to assist you.