Butter is basically made of animal fats which contain saturated fatty acids. Cooking oil on the other hand, contains long unsaturated fatty acids. This means that vegetable oils can be hydrogenated whereas butter cannot be hydrogenated.

A reaction in which one atom or a group of atoms is replaced by another atom or another group of atoms is called substitution reaction. For example, consider the photochemical reaction of methane with chlorine. In the presence of sunlight, one of the H atoms is replaced by Cl atom. Gradually, each H gets replaced by Cl. The reaction can be represented by the equation:

CH₄ + Cl₂→ CH₃Cl + HCl (in the presence of sunlight)

CH₃Cl + Cl₂→ CH₂Cl₂ + HCl and so on.