Why do salt and sugar readily dissolve in water and not in oil?

    The nature and the degree of interaction between the solute and the solvent decide whether or not a solution will form. For the solute to dissolve, the particles must break from one another and move into the solvent.

At the same time, the solvent particles must allow the solute particles to come between them. In other words, the attractive force between solute and solvent particles must be greater than solute-solute particles as well as solvent-solvent particles, to allow a solution to form.

            A general rule for predicting the dissolution is “like dissolves in like”. Substances are classified into three types, namely polar (capable of showing +ve and _ve poles within the molecule, e.g.: water and hydrochloric acid), nonpolar (e.g.: vegetable oils, petroleum oils petrol, diesel, etc, and iodine) and ionic (capable of splitting into ions like cation and anion, e.g.: salts like NaCI and KCI).

Polar solvents can dissolve polar and ionic solutes but not nonpolar solutes. Likewise, nonpolar solvents can dissolve nonpolar solutes but not the other solutes. Sugar is a polar covalent compound and salt (common salt, NaCI) is an ionic compound. Both of them can dissolve in a polar substance like water, but not in a nonpolar substance like oil. Oil and water are always immiscible (insoluble in one another) because they are dislike substances.

It is worth remembering that sugar and salt are obtained from sugar-cane juice and sea-water respectively, which arc their corresponding solutions, where the solvent is water. Hence there is nothing strange in their dissolution in water. Similarly, wax is obtained from petroleum oil, as a fraction. It does dissolve in petroleum or any of its other fractions such as petrol, ether, pentane, benzene, etc. It can he noted that in oil wells, water and oil are found to be present together, but as separate layers. Vegetable oils also form separate layers with water.

We need not expect water and oil to behave alike. They differ already from each other in number of respects. They do not have any common properties at all, except that they are liquids at room conditions and are consumed (only edible oils) by us.

 In short, we can understand that water and oil are not comparable substances.