On a hot day, the sun causes the sidewalk to become very warm. Which is a valid claim about how the particles that make up the sidewalk change as the temperature increases?

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).