Question

Define physical and chemical adsorption with suitable example

Answer 1

Physical Adsorption

• Physical adsorption, or physisorption, is a type of adsorption that occurs for most particles in contact with a solid or liquid surface. Condensation of water molecules sticking to a drinking glass is an example. An extremely cold glass will build up many hundreds or thousands of layers of water molecules because each new layer of water molecules can stick to the previous layer. It's not necessary for incoming water molecules to ''find a spot'' on the glass surface in order to stick.
• The top of the glass in the picture is open to air and slightly warmer. As a result, fewer water molecules (which are not drawn to scale) can stick to the top part of the glass.
• If you warm the glass back up, you'll notice the condensation dripping off because the physisorption gets weaker as atoms in the glass jiggle around at higher temperature. As a result, water molecules occasionally get an extra jolt of energy--just enough to free them from their sticky neighbors and melt off the glass. In other words, the molecules desorb from the surface. Desorption is the opposite of adsorption.

Chemical Adsorption

• Chemical adsorption, or chemisorption, is characterized by chemical interactions that can range from very weak to at least 100 times stronger than in physisorption. In chemical adsorption (as the name implies) there is some chemistry involved. In other words, chemical bond formation is required. Some molecules can chemisorb to a given surface, but others cannot.