Introduction
Absorption is the process in which a fluid (absorbate) is taken in by a material (absorbent). That is, the fluid soaks through/penetrates the absorbent. Adsorption refers to the process by which the atoms, ions, or molecules of a matter (gas, liquid, or dissolved solid) stick to the surface of a solid. Most people are bewildered when they hear the word ‘adsorption’, and that is understandable. After all, not everyone is a science enthusiast. Adsorption seldom occurs without preceding absorption.
On the other hand, absorption is a well-known phenomenon. How many times a day do people mop up a mess? That’s right, too often. A house never seems to be clean when kids are around (but one can’t begrudge the kids their antics!). A sponge soaked in water, a tissue paper used to blot ink, and a suit ruined by accidentally spilled wine are all examples of absorption.
The term ‘sorption’ includes absorption and adsorption; the reverse of sorption is defined as desorption. Desorption is much more challenging to achieve when absorption has occurred. On the other hand, desorption is a simpler process when adsorption occurs. An increase in temperature speeds up the desorption process.
Absorption Vs. Adsorption
The main difference between absorption and adsorption is in the former process, the absorbate permeates the absorbent, whereas the latter is defined as the adhesion of the adsorbate fluid’s molecules to the adsorbent.
Difference Between Absorption And Adsorption In Tabular Form
| Parameters Of Comparison | Absorption | Adsorption |
| Phenomenon | It is a bulk phenomenon. The absorbate penetrates through the core of the absorbent solid. | It is a surface phenomenon. The atoms, ions, or molecules do not penetrate the surface. |
| Precession and Succession | Absorption frequently occurs after adsorption. | Adsorption often precedes absorption. |
| Endothermic or Exothermic | It is an endothermic process. | It is an exothermic process. |
| Concentration | The concentration is consistent/uniform throughout, as the absorbate penetrates through the surface, and the entire volume of the absorbate is soaked. | The adsorbate is concentrated on the surface, and only a minimal amount penetrates the surface. Therefore, the concentration is high on the surface and negligible beneath the surface. That is, the concentration is not uniform. |
| Ease of Separation | Once the molecules are assimilated into the solid, liquid, or gas absorbate, it is nearly impossible to separate them. | Separating the molecules from the solid is relatively easy, as they are only stuck to the surface. |
What Is Absorption?
Absorption is the process in which one matter permeates another matter in a different state (liquid, solid, or gas). That is, a substance’s molecules get completely dissolved or assimilated into another matter. The absorbent’s energy increases when it absorbs, as the absorbate’s energy adds to its existing energy. One way to immediately identify whether absorption or adsorption has taken place is to check whether a thin film of one substance’s molecules clings to the other material’s surface. The film formation is an identifying mark of adsorption; it never occurs in absorption.
Types Of Absorption
Chemical Absorption
Chemical absorption is also known as reactive absorption, as the process involves chemical reactions between the substance being absorbed and the matter absorbing it. At times, combined chemical and physical absorption takes place. An example of chemical absorption is food digestion, which results in energy production.
However, light absorption is a far more intriguing example. In this process, light is converted into energy. Each matter or material has electrons with a specific vibration frequency (scientifically known as natural frequency). When a light wave with the same frequency hits the matter, the energy is absorbed. Apparently, that is why various objects have different colors. Atomic absorption spectroscopy and molecular absorption spectroscopy are the techniques used to measure the energy absorbed during light absorption.
Sound absorption or acoustic absorption is another example worth looking into. A sound wave may be reflected, transmitted, or absorbed by a medium. Sound absorption is the removal of energy from a sound wave when it passes through or strikes a medium. The absorbed sound is converted into heat energy. The most well-known medium through which sound travels is air. Acoustic absorption is an integral part of soundproofing. Porous materials are ideal for absorbing sound. Damping (reduction of sound resonance) involves absorption (to reduce overall sound) and redirection.
Physical Absorption
Physical absorption does not involve any chemical reaction. Hydrophilic (molecules attracted to water molecules) solids are highly effective in absorbing water. A common example of physical absorption is oxygen absorption by water. The absorption of carbon dioxide by water is another excellent example.
What Is Adsorption?
Simply put, Adsorption is the adhesion of the ions or molecules of a substance to the surface of a solid matter. Protein adsorption is an intriguing phenomenon. Proteins are composed of charged amino acid subunits. This charge allows proteins to stick to other molecules’ surfaces. Activated carbon is the best example of an adsorbent; it is used to treat waste gas and water and is effective in adsorbing organic adsorbates. The first stage in a virus’ life cycle is adsorption. The types of adsorption are as follows:
Types Of Adsorption
Chemisorption
In chemisorption, a chemical reaction takes place between the surface and the absorbate resulting in new chemical or electronic bonds. Heterogeneous catalysis, a well-known example of chemisorption, involves a cycle of absorption, reaction, and desorption (the reversal of sorption) at the absorbate substance’s surface.
In heterogeneous catalysis, the adsorbent is the catalyst (a substance added to increase the rate of a chemical reaction), and the reactant (substance consumed during a chemical reaction) is the adsorbate. Molecular adsorption is a type of chemisorption in which the adsorbate remains intact after binding to the adsorbent material’s surface. Dissociative chemisorption involves the breakdown of the adsorbate substance’s bonds, which combine with the molecules of the adsorbent matter’s surface to form new bonds.
Physisorption
Physisorption or physical adsorption does not result in any change in the adsorbate molecule structure. Instead of chemical bonds, it is the Van der Waals force that causes the bond between the absorbate and the adsorbent. The binding energy in physisorption is weak compared to chemisorption. Geckos (a type of lizard) have the ability to climb vertical walls due to physisorption (in layman’s terms, their foot hairs stick to the wall’s surface).
Masks, especially miners’ masks, are the best example of physisorption. The activated carbon in the masks adsorbs dust particles and any harmful gases the wearer may come across. In addition to the absence of chemical reactions, another factor that distinguishes physisorption from chemisorption is the former process results in the formation of multiple layers of adsorbed particles, while the latter causes the formation of only a single layer.
Water adsorption plays a vital role in chemical engineering and catalysis. It is easy to reverse this phenomenon. The water that is stuck to the adsorbent’s surface can be eliminated merely by drying it at the required temperature or by vaporizing it. Another example of adsorption is a misty window. The water vapors (adsorbate) stick to the windowpane’s surface. Moreover, have you ever wondered why a glass pane frosts over when one blows on it during a chilly night? The answer is Adsorption!
Adsorption Spill Over
Spillover occurs when the adsorbed adsorbate moves and accumulates on a different surface. Hydrogen spillover is the most common example of adsorption spillover. It occurs during heterogeneous catalysis. In this process, the hydrogen atoms separate from a metal surface and migrate to a non-metal surface.
Main Difference Between Absorption And Adsorption In Points
- Energy is absorbed in the form of heat when absorption takes place (as it is an endothermic process), whereas heat is released during adsorption, resulting in the rise of the surrounding’s temperature (exothermic process).
- Adsorption does not depend on absorption; however, absorption often takes place after adsorption.
- In absorption, the concentration of the absorbate in the absorbent is uniform (the entire volume and not merely the absorbent’s surface is permeated). On the other hand, the concentration rate is not uniform in adsorption, as the fluid never penetrates the absorbent’s surface.
- Adsorption helps in solar heating and storage, adsorbing organic substances, and is an essential process in chemical energy. Absorption of carbon dioxide results in the carbonation of beverages, which prevents spoilage (also, who does not love the soothing tangy flavor?)
Conclusion
Briefly, adsorption and absorption are simply referred to as sorption. Remember, if the molecules of one substance penetrate the surface of the others, it is absorption. If not, and the molecules merely stick to the surface, it is adsorption. It is not a complex concept to wrap one’s mind around. Sorption and desorption are processes people often encounter in their lives without realizing it.
It’s funny how complex sounding concepts like light or sound absorption and chemical adsorption are, in reality, quite interesting and easy to understand concepts. Anyway, knowing the difference between absorption and adsorption provides an opportunity for people to mess around with their friends. Imagine the confusion on their faces when they hear the words absorbate, absorbent, desorption, and so on. Going through the trouble of learning the differences is so worth those looks, right?
