Adsorption: Definition & Examples

Instructor: Justin Wiens

Justin teaches college chemistry and has Bachelor and Doctorate degrees in chemistry.

In this lesson, we will discuss the process of adsorption/desorption and contrast it to the process of absorption. We will discuss several examples of each process.

Introduction to Adsorption/Desorption and Absorption

Surfaces. They're all around us, from your kitchen table to the sidewalk outside to the alveoli in your lungs. Surfaces separate one chemical phase—solid, liquid, or gas—from another one. Many physical and chemical processes occur either at the interface between a surface and the surrounding phase or phases, or in the phases themselves. Let's talk about the precursors to these processes: adsorption, desorption, and absorption.

Adsorption

Adsorption is the sticking of a particle to a surface. The particle can be a gas, liquid, or solid atom, molecule, or ion, and the surface can be a liquid or solid. Adsorption is constantly happening all around us. In fact, molecules of water, oxygen, and other atmospheric gases are continually adsorbing to the surface of your skin, your kitchen table, the walls around you---any surface a particle can hit, it can adsorb!

When a particle encounters a surface, it can either bounce off of the surface (like a billiard ball bouncing off the side of a pool table) or adsorb to it. Whether the particle adsorbs depends on whether it can dump excess energy to its surroundings: the surface it hits). Any chemical attraction between the surface and the particle also increases the likelihood of adsorption.

Desorption

Atoms and molecules are constantly moving and also have some amount of energy, more so as their temperature is increased. As the atoms at or near a surface jiggle around, sometimes they can give the adsorbing particle on the surface a ''kick'' of energy, causing it to leave the surface. This process is called desorption.

This image shows the difference between adsorption and desorption

If we think about it, desorption is heavily favored when the surface is at a much higher temperature than the adsorbing particle. In this case, even if the particle is attracted to the surface--for example, water molecules from the air easily adsorb to glass--the particle's desire to stick cannot compete with the surface's desire to kick. That's why condensation easily forms on a cold glass, but quickly evaporates from a hot one.

Absorption

A related process to adsorption is absorption, which is the process of a particle going from one phase into a liquid or solid phase, rather than onto a surface.

Most of us are familiar with paper towel commercials where the advertiser's brand is apparently more absorbent than the average competitor's. In other words, some paper towels absorb more liquid than others.

paper towels

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