Verified answer

expands on heat, contracts on cold.

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Metals expand when heated and contract when cooled. A few will contract upon melting. These include:

Si, Ga, Ge, Bi, Sb and GaAs

Metal atoms, like all atoms, are constantly in motion (this means they have kinetic energy). That is actually what temperature is, a measure of the motion of atoms in a substance. When you heat up metal the atoms start moving move (gain more kinetic energy). This extra movement causes the material to expand becuase the atoms are now farther apart, "bouning off" of each other more.

The opposite happens when metal is cooled down, it strinks becuase the less kinetic energy allows the bonds between the atoms to pull the atoms close together.

All of this is assuming you are staying in solid form, werid things can happen while changing states (melting etc).

Most things expand when heated, shrink when cooled, metals follows this trend. The biggest outlier for this is water which actually expands when frozen....