Silica gel pellets, have you ever heard this name? You must have seen a white colored packet in your new bag that has “Throw away. Do not eat” written on it. It is a packet of silica gel pellets and it is used for dehumidification. Dehumidification is a process in which water vapor present in the air or the surroundings is removed. Water as we know is essential for the existence of life; but you don’t want your new bag to be infested with fungus, so to avoid it both liquid and vapor forms of water is removed from the bags and to achieve the latter silica gel is used. Let’s discuss how silica gel can remove water vapor.
The chemical nature of the surface of a solid is different from its core. The atoms or molecules on the surface of the solid bind with the core on one side but on the other side they are free and active. This is similar to the concept of evaporation which we learnt during our Physics tuition classes. These molecules or atoms, in the case of the solid, can attract chemical species present in their immediate surrounding. The effective electric field of the charged particles present in the atoms and molecules attracts the molecules or atoms present in the surroundings. This process of attracting and amassing the chemical species on the surface of a solid is termed as ‘adsorption’.
Adsorption is a surface phenomenon unlike absorption that occurs at the bulk of the material. Depending on the nature of attractive forces involved adsorption is classified into chemisorption and physisorption. The solid species that adsorbs is called as adsorbent and the species that is adsorbed is called as adsorbate. The efficacy of the adsorbent depends on the surface area of it, which is unoccupied. It also depends on temperature and the pressure of the adsorbate or the concentration of the adsorbate etcetera.
The reverse of adsorption is called as desorption. Both adsorption and desorption occur simultaneously and depending on the rates of each of these, the overall process would be either adsorption or desorption. Hence the process is dynamic i.e, even when you do not see any considerable change in the concentration of the adsorbate on the surface, both adsorption and desorption might be occurring at a very rapid rate. Silica gel pellets adsorb water vapor present in the atmosphere and thus act as dehumidifier. Adsorbents like activated charcoal are used to remove color causing agents from a solution.