Why is the planet Uranus so cold?

Because of the far distance from the sun, the farther away, the colder

Uranus is so cold because it is too far away from a heat source (aka the sun)

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).