How could knowing the melting point of a substance be helpful to
scientists?

There going to know when is going to freeze

As cooking food that has to go in the oven

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).