Deliquescence

Some substances can change the body structure if kept in an open area. This is due to the absorption of water vapour or the release of water molecules from its structure. There is about 0-4% water vapour in the air, depending on the location and time of day. Solids can be dissolved by absorbing water vapour but this absorption depends on the humidity of the environment. 

Fluids are crystals that can lose water molecules that already exist in their cellular structure. Hygroscopic objects are another type of solid object that can absorb or emit vapour from the atmosphere. But these substances do not dissolve after absorption. 

Glossy, soluble and hygroscopic compounds are terms used in chemistry to describe chemical changes that occur in a structure during a chemical reaction. These mutations reflect the type of bond that exists in the reactors, their oxidation numbers, their electron affinity and their ability to remove other elements. The combination of efflorescent, deliquescent and hygroscopic is also an indication of the ability of shiny absorbing compounds.

The main difference between glossy and hygroscopic materials is that the glossy creates an aqueous solution by absorbing water vapour and is less likely to evaporate and hygroscopic materials can absorb water vapour but do not form an aqueous solution.

Example of Deliquescent: Sugarcane substances can absorb high amounts of water vapour.

Example of Efflorescent: Floral material does not degrade water vapour.

Define Deliquescence

Deliquescence is the process by which an object absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution. Certain substances have the property to absorb moisture when exposed to the atmosphere at ordinary or at any temperature, they initially become wet, lose their crystalline form and finally dissolve in water to form a saturated solution. 

Deliquescent meaning can be said as 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. Deliquescent substances are solids that tend to absorb moisture from the air and dissolve it. For example, 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 crystallisation molecules 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. 

We can say Efflorescence, the automatic loss of water by aqueous salts, which occurs when the pressure of the hydrate vapour is greater than the partial pressure of the air vapour. For example, Na₂SO₄.10H₂O, FeSO₄.7H₂O, Blue Vitriol (CuSO₄.5H₂O), washing soda (Na₂CO₃.10H₂O) lose 9 water molecules first and then become completely anhydrous.

When sodium hydroxide pellets and washing soda crystals are found in an airtight container, concentrated tetraoxosulphate (VI) and each quicklime are placed in a glass after carefully observing its appearance. When you leave each of these objects in the open for a moment, you would notice a marked difference in their appearance that indicates their ability to lose or absorb moisture from the atmosphere.

The efflorescent substance is a water-based chemical compound with its molecules, which, when exposed to air, lose this water through evaporation. A common example of this action is cement drying.

Hygroscopy

Certain substances absorb moisture from the atmosphere at ordinary temperatures but do not dissolve in it. These are called hygroscopic substances, and the property is known as hygroscopy. 

A hygroscopic object will absorb moisture from the atmosphere but will not form a solution. It can only stick if it is strong. Hygroscopic fluid such as concentrated tetraoxosulphate (VI) acid will absorb water from the atmosphere and can usually purify itself three times its original volume. Hygroscopic materials are very useful as drying agents in the laboratory. Some examples of other hygroscopic substances are (i) Sodium trioxonitrate (V) 

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 change in viscosity. Also, certain liquids absorb water from the air for purification, and these are also considered hygroscopic. For example,  CaO, NaNO₃, sucrose CuO. conc. H₂SO₄  and conc. HCl.

Zinc chloride, sodium chloride, sodium hydroxide crystals, silica gel, honey, nylon, and ethanol are hygroscopic in nature. When a hygroscopic object absorbs so much moisture that a liquid solution is formed, the object becomes desolate.

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.

Drying Agents

Drying agents are substances or compounds that have a strong mixture of water or moisture. These items can be deliquescent or hygroscopic. They are commonly used to dry gas in the laboratory and are often used in desiccators. It should be noted that the drying agent cannot be used when reacting to a drying object. For example, concentrated tetraoxosulphate (VI) cannot be used to dry a compound like ammonia as it can react with the formation of ammonium tetraoxosulphate (VI).

 2NH₃ (g) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)

Difference between Efflorescent Substances, Deliquescent Substances, and Hygroscopic Substances

Did You Know?

In the rainy season, you might have noticed that the salt turns sticky. This is due to the fact 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.