Explain why for static electricity to occur between two surfaces that are rubbed against each other, one of the materials has to be an insulator

The rubbing creates a negative charge that is carried by electrons. The electrons can build up t produce static electricity.

They can measure  the amount of energy gained or lost

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).