Why do cats purr when they are happy?

Cats use the ability to purr for more than just that they use it when they are frightened, when they want other cats to know that they are friendly,and when its mealtime. but if your asking how they purr its just something in there brain telling them to purr.

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).