How does the movement of molecules inside a solid block of iron change as the iron is heated?
the molecules move faster.
the molecules move slower.
the molecules stop moving completely.
the molecules move in the same way at all temperatures.

The correct answer among the choices is the first option. The movement of the molecules inside a block of iron when it is heated is affected by the movement being faster as compared to the first conditions. Temperature is said to be directly proportional to kinetic energy which means as temperature increases, the kinetic energy also increases. Therefore, molecules now are moving faster.

The mass has escaped into a gas which you can not see so the law stays true.