Adsorption and Absorption: The two most common phenomena in surface chemistry are adsorption and absorption. Although adsorption and absorption sound very similar, they are very different from each other. The absorption process is defined as a mechanism in which particles in one state of matter are transferred to a substance in another state of matter. Whereas in adsorption, the molecules are held loosely on the surface of the adsorbent and can be easily removed.

The basic difference between adsorption and absorption is that adsorption is a surface phenomenon while absorption is a bulk phenomenon. The examples of adsorption and absorption involve the adsorption of water vapours on silica gel in air conditioners and the absorption of water in a sponge. In this article, we have explained the types, the difference between adsorption and absorption, and their examples in detail.

What are Adsorption and Absorption?

Adsorption and absorption concepts are explained in detail below:

Define Adsorption

Adsorption is a phenomenon in which particles in the form of atoms, molecules, or ions form a thin film on the surface of another substance on which it gets adsorbed. The solid surface attracts liquid or gas molecules towards itself to adhere to them when coming in their contact. Some basic terms commonly used in adsorption are:

1. Adsorbate – The substance whose particles (atoms, molecules, or ions) get adhered to on the surface is known as adsorbate.
2. Adsorbent – The surface where these adsorbates get adhered, i.e. where adsorption takes place is known as the adsorbent. Examples of adsorbents are silica gel, alumina gel, activated charcoal, graphite, etc.

Define Absorption

Absorption is a phenomenon in which particles in one state of matter are transferred to a substance in another state of matter. Like absorption of gases on liquids or solids, absorption of liquid on solid. Some basic terms commonly used in adsorption are:

1. Absorbate – The substance that has to be soaked up or absorbed on another substance is known as absorbate.
2. Absorbent – The surface where these absorbates get absorbed. In short, it is a material that can soak up liquid easily is known as absorbent.

Adsorption and Absorption Difference

Adsorption	Absorption
Adsorption is defined as a phenomenon in which atoms, molecules, or ions of a substance, get adhered to the surface of the adsorbent.	Absorption is defined as the phenomenon in which a liquid substance, known as absorbent, gets soaked or absorbed completely into the surface of the absorbent.
It is a surface phenomenon.	It is a bulk phenomenon.
The concentration of the adsorbent is not uniform; it decreases from the surface to the bottom of the adsorbent.	The concentration of adsorbate molecules is uniform throughout the absorbent.
The rate of the reaction steadily increases and reaches equilibrium.	The rate of the reaction is uniform throughout the reaction.
Adsorption is an exothermic process.	Absorption is an endothermic process.
Example: Adsorption of water vapours on silica gel in air conditioners.	Example: Absorption of water in a sponge.

Example of Adsorption and Absorption

The examples of adsorption and absorption are explained below:

Examples of Adsorption

1. Adsorption of water vapor on silica gel reduces humidity.
2. Adsorption of ammonia molecules on activated charcoal
3. Gas masks used in coal mines adsorb poisonous gases and prevent the workers from inhaling them.
4. In chromatography, the mixture is passed through the adsorbent bed and impurities are separated to obtain a pure compound.
5. Many drugs are adsorbed to kill germs in most antiseptics and disinfectants.
6. In the paint industry, dissolved gases in paints are removed by adsorption because, if gases are not removed completely, paints will not adhere properly to the walls.
7. Sugar is decolorised from activated charcoal.
8. The cleansing action of soaps and detergents is a common example of adsorption.

Examples of Absorption

1. Water gets absorbed on calcium carbonate.
2. The absorption of oxygen in water is responsible for the survival of the aquatic ecosystem.
3. Absorption of water in the sponge
4. Hydrogen sulfide is removed from biogas by absorption. In this process, firstly hydrogen sulfide is absorbed by water and then reacted with iron oxide.

Adsorption and Absorption Takes Place Simultaneously

When both adsorption and absorption take place simultaneously, another phenomenon occurs which is known as ‘sorption’. Thus, sorption involves the accumulation of molecules on the surface of solid or liquid as well as soaking up the molecules inside the bulk of a solid or liquid. Sorbate is the substance that gets attached towards the sorbent (surface on which sorption takes place). Examples of sorption are:

1. Dipping of sponge in water. In this process as soon as we dip the sponge in water, firstly water molecules form a film on the layer of the sponge and secondly, water molecules start getting inside the bulk of the sponge and in this manner, both adsorption and adsorption occur simultaneously.
2. Cotton fibers are dyed by azo dyes by the process of sorption.

Types of Adsorption

Based on the types of bonds formed between adsorbate and adsorbent molecules, adsorption is divided into two types: physisorption and chemisorption.

1. Physisorption – It is a type of adsorption in which a weak physical bond due to Van Der Waals force is formed between gas molecules of adsorbate and solid or liquid molecules of adsorbent. In this type of bonding, no chemical change occurs and the electronic structure of the atoms and molecules are not perturbed. For example, hydrogen gas gets adsorbed on the surface of charcoal.
2. Chemisorption – It is a type of adsorption in which strong chemical bonds are formed between the adsorbate and the surface of the adsorbent. In chemisorption, the electronic structure of the atoms and molecules gets perturbed. For example, corrosion.

Types of Absorption

Based on the types of bonds formed between adsorbate and adsorbent molecules, absorption is divided into two types: Physical absorption and Chemical absorption.

1. Physical absorption – It is an absorption process in which no reaction occurs between the molecules of absorbate and absorbent. Weak physical bonds are responsible for absorption. For example, absorption of oxygen gas in water. 
2. Chemical absorption– It is an absorption process in which a chemical reaction occurs between the molecules of absorbate and absorbent. The best example of chemical absorption is the digestion of food.

Examples of Adsorption in Daily Life

Some applications of adsorption in our day to day life are:

1. Silica and aluminium gels are used in air conditioners to absorb moisture in the air to reduce humidity.
2. Using charcoal as an adsorbent, noble gases can be separated.
3. One of the most common uses of adsorption is the cleansing action of soaps and detergents; they adsorb the dirt on the surface of the clothes.
4. Adsorption chromatography is one of the most widely used applications for the purification of chemical compounds in chemical laboratories.
5. Gas masks used by workers in coal mines work on the principle of adsorption. They adsorb poisonous gases in coal mines and let the pure gases enter inside the body of workers.

Examples of Absorption in Daily Life

There are numerous examples of absorption in our daily life. Some of them are listed below:

1. Absorption of light is a very common phenomenon, when sunlight falls on the grass, it absorbs red, orange, yellow, blue, indigo, and violet light, and only green colour is reflected. Hence, the colour of the grass appears green.
2. A black surface appears so because it absorbs all the colours i.e., red, orange, yellow, green, blue, indigo, and violet light.
3. Milk spilled in the kitchen is absorbed by using a towel.
4. Water-soaked by paper towels is an example of absorption.
5. When chalk is dipped in ink for some time, it takes the colour of ink due to absorption.
6. Oxygen and carbon dioxide are dissolved in water due to absorption.
7. Absorption of ammonia gas by water
8. Absorption of digested food by blood in our body and then these nutrients from food is transported to rest of the body parts.
9. Absorption of drugs and medicines by our bloodstreams.
10. Dermal absorption i.e., absorption of sunlight through the skin.

Summary

In a nutshell, it can be said that both adsorption and absorption are a part of surface chemistry but they differ in their mode of action. Adsorption is a phenomenon in which adsorbate molecules get attracted towards the surface of the adsorbent while absorption is a phenomenon in which absorbate molecules get completely soaked up inside the absorbent.

Thus it can be concluded that adsorption is a surface phenomenon and absorption is a bulk phenomenon. Various technologies are invented on these processes like adsorption of refrigeration systems and further research is going on to develop these surface phenomena for further use.

FAQs on Absorption and Adsorption

Q.1: What is the difference between adsorption and absorption? Give one example for each?
Ans: Adsorption is defined as a phenomenon in which atoms, molecules, or ions of a substance, get adhered to the surface of the adsorbent. It is a surface phenomenon. Absorption is defined as the phenomenon in which a liquid substance, known as absorbate gets soaked or absorbed completely into the surface of the absorbent. It is a bulk phenomenon. For example, the adsorption of water vapours on silica gel and water-soaked by paper towels is an example of absorption.

Q.2: What is the difference between absorption & adsorption and sorption?
Ans: Absorption is a phenomenon in which particles in one state of matter are transferred to a substance in another state of matter. Adsorption is a phenomenon in which particles in the form of atoms, molecules, or ions form a thin film on the surface of another substance on which it gets adsorbed and when both adsorption and absorption take place simultaneously, another phenomenon occurs which is known as sorption.

Q.3: What is called adsorption?
Ans: Absorption is defined as the phenomenon in which a liquid substance (adsorbate) gets soaked or absorbed completely into the surface of the absorbent. It is a bulk phenomenon.

Q.4: What is the relation between adsorption and absorption?
Ans: The basic relation between adsorption and absorption is that both are surface phenomena. Adsorption deals with the surface and absorption deals with the entire bulk of the substance on which it happens.

Q.5: What is the function of absorption and adsorption?
Ans: Absorption is a process of uptake of substances such as nutrients by the cells and tissues in our body by diffusion or osmosis whereas; adsorption is the process of sticking adsorbate molecules on the surface of the adsorbent such as a solid surface.

Q.6: The phenomenon of simultaneous absorption and adsorption is called?
Ans: When both adsorption and absorption take place simultaneously, another phenomenon occurs which is known as sorption.

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