Introduction
Absorption and adsorption are two of the most discussed topics in analytical chemistry. However, unlike their spellings, the two phenomena have quite different meanings, but their primary difference is their driving force. To put it in simple words, absorption means to take something into another substance, and adsorption is the attraction of molecules onto the surface of a solid. The substance that is absorbed is termed absorbate, and the substance that absorbs is termed absorbent. However, in the case of adsorption, the terms are named adsorbate and adsorbent, but the meaning is the same.
Absorption vs. Adsorption
Absorption is a bulk phenomenon defined as the phenomenon in which a liquid substance, known as absorbent, gets absorbed completely into the surface of the absorbent. Adsorption is a surface phenomenon defined as a phenomenon in which atoms, molecules, or ions of a substance hold to the surface of the adsorbent. Depending upon various factors, both phenomena follow different ways to proceed and, therefore, have different applications. One thing that is common between the two phenomena is that these are sorption processes, which means connecting one substance to another.
Difference Between Absorption and Adsorption in Tabular Form
| Absorption | Adsorption |
| Absorption is the process of completely absorbing the substance. | Examples soaking water with paper towels, dipping chalk in ink, ice production and cold storage. |
| Absorption is a bulk phenomenon. | Adsorption is a surface phenomenon. |
| Absorption is an endothermic process. | Adsorption is an exothermic process. |
| Absorption is not affected by temperature. | Adsorption is dependent on temperature. This process is mostly seen at low temperatures. |
| Absorption is independent of pressure. | Adsorption is dependent on pressure. |
| There is no such chemical interaction involved between absorbate and absorbent. | The adsorbate sticks to the adsorbent via Vander wall forces. |
| The concentration of the absorbed material is uniform throughout the absorbent. | The concentration of the adsorbed material is high on the surface of the adsorbent. |
| Examples: soaking water with paper towels, dipping chalk in ink, ice production, and cold storage. | Examples are the adsorption of a gas by activated charcoal, the adsorption of air by silica gel, water purification, and chromatographic analysis. |
| Absorption depends upon the surface area | Adsorption is independent of surface area. |
What Is Absorption?
Absorption is a chemical or physical phenomenon in which the absorbate (absorbed material) penetrates the bulk phase, i.e., solid/liquid/gas of the material in which it is taken up. In this process, one substance gets mixed or absorbed into another substance. The substance which is absorbed is termed the absorbate, and the substance which absorbs is termed the absorbent. This particular phenomenon occurs on the whole body and is therefore referred to as the bulk phenomenon. Examples of absorption include petroleum refining and selective dissolution. Absorption occurs in all living and non-living organisms to intake nutrients and maintain internal stability according to the changing environment.
Applications
- Separation of the gas mixture: This is done by absorption of a part of the mixture by a solvent.
- Glycol dehydration of raw natural gas: by absorption of water vapor contained in the natural gas.
- Purification of natural gas: by passing through a solution of ethanolamine.
- Removal of H2S gas: by contacting the hydrogen sulphide with a bed of solid zinc oxide with which it reacts to form zinc sulphide.
Types of Absorption
There are two types of absorption:-
- Physical Absorption
- Chemical Absorption
Physical Absorption
This type of absorption is non-reactive. The process depends on parameters such as the solubility of the gases, temperature, and pressure. Following Henry’s Law, the solvent capacity in this process increases linearly with an increase in the pressure, and the solvent is regenerated by reducing the pressure. The process is reversible in nature.
- Examples of physical absorption include the dissolving of oxygen in the air into the water and the separation of propane and butane from a hydrocarbon gas mixture.
Chemical Absorption
Unlike physical absorption, a chemical process is reactive and irreversible in nature. In this process, chemical reactions occur between the absorbent and the absorbent. However, in some cases, this process occurs in combination with physical absorption. The stoichiometry of the reaction and the concentration of the reactants are some factors on which the process depends.
- Digestion of food is the most common example of this kind of absorption.
What Is Adsorption?
In this phenomenon, one substance is accumulated on the surface of another substance. The substance that accumulates on the surface is termed adsorbate, and the substance on which the adsorption occurs is termed adsorbent. The adsorbate can be a gas or a liquid, while the adsorbent is usually a solid. It is a surface phenomenon, which means that the phenomenon is restricted to the surface only and not the whole body, as in the case of absorption. Adsorption is mainly found in physical, chemical, and biological systems, but it also has various industrial applications. Adsorption is used in separation techniques like ion exchange chromatography, in which the molecules are separated by selective transfer of the liquid phase to the surface of the solid. The most common use of adsorption is for the removal of non-biodegradable compounds from the groundwater.
Applications
- Dyeing: The cloth is first dipped in a mordant, then a dye solution.
- Gas masks: These use suitable absorbents to protect us against gases such as chlorine, methane, and sulphur dioxide.
- Noble gas separation: by adsorbing these gases on activated charcoal.
- Sugar clarification: Charcoal powder is added to the sugar solution, which adsorbs every unwanted color.
Types of Adsorption
There are two types of adsorption:-
- Physical adsorption
- Chemical adsorption
Physical Adsorption
It is also known as physisorption. In this process, the molecules are attached to the surface Via weak Vander wall forces, hydrogen bonding, or hydrophobic interactions. Therefore, it is a weak phenomenon. It is also reversible in nature. Physisorption depends on parameters such as the nature of the adsorbate, surface area, temperature, and pressure.
- For example, charcoal adsorbs H2 and N2 gas.
Chemical Adsorption
Also known as chemisorption, this process is accomplished by the sharing of electrons between the adsorbate and the adsorbent to form a strong covalent or ionic bond. Therefore, unlike physisorption, it is a strong phenomenon and irreversible in nature. Factors such as surface area, temperature, and the nature of the adsorbate also affect chemisorption.
- Examples of chemisorption include the adsorption of hydrogen, etc., on the surface of ferrous catalysts at a high temperature.
Main Differences Between Absorption and Adsorption in Points
- Absorption involves the mass transfer of particles into another material, and adsorption takes place by sticking one particle over another.
- Absorption is a bulk phenomenon, while adsorption is a surface phenomenon.
- Absorption is an endothermic process, whereas adsorption is an exothermic process.
- Temperature has no effect on absorption, while adsorption is highly affected by temperature. Adsorption is favored at low temperatures.
- The concentration of the absorbed material is even throughout the material, while the surface concentration of the material adsorbed is more than that in the internal case of adsorption.
- Absorption has a uniform rate of occurrence, whereas adsorption steadily increases and reaches an equilibrium.
- Absorption is independent of pressure, while adsorption is dependent on pressure.
- Absorption depends upon surface area, while adsorption does not.
Conclusion
To be precise, absorption and adsorption, no matter how similar they may sound, have completely different meanings and, therefore, completely different applications. Absorption is a bulk phenomenon in which the absorbate occupies the whole volume of a substance via no such release of heat, whereas adsorption is a phenomenon strictly restricted to the adsorbent’s surface with the release of heat. Both these processes have various applications in real life, such as froth flotation, noble gas separation, purification of water, manufacture of vitamin B1, and many more.
