A. As for the difference between absorption and adsorption, the former is a bulk phenomenon, while the latter a surface phenomenon. Basically what I mean to say is, take the example of a cloth being immersed in water. Water is absorbed by the cloth. But dust on cloth is adsorption, as the dust doesn't enter between the fibre molecules, rather stays on the surface of the cloth.
Exactly is the case with anhydrous calcium chloride and silica gel. CaCl2 is naturally a hydrated salt, of the type CaCl2.2H2O or CaCl2.6H2O. When in anhydrous condition, it readily 'absorbs' water molecules to transform into its hydrated state. So it is used in the preparation of CaCO3.
On the other hand, silica gel is an excellent adsorbent and the air becomes dry in its presence because the water becomes 'adsorbed' on its surface.
B. The given sol is positively charged. In order to bring about the coagulation oppositely charged ions must be added.
According to Hardy Schulze rule, greater the charge on the oppositely charged ion of the electrolyte added, more effective is the coagulation. Thus to coagulate a positively charged colloid, we must see the negatively charged ions.
H2SO4 contains SO4-2ions and H3PO4 contains PO4-3ions.
Thus PO4-3ions bearing the higher negative charge is more effective in bringing the coagulation of positively charged sol.
C. Sulphur sol formed by large number of atoms and molecules which generally have diameters less than 1mm hence it is multimolecular colloid. Protein have large size hence macromolecular colloid.
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