Question

why does the volume of a substance expand when it gains heat/energy? because its molecules move further apart because its molecules move closer together

Explanation:

When a substance gains heat/energy, its molecules gain kinetic energy. This increased energy causes the molecules to move more vigorously. In most substances (except for some exceptions like water in a certain temperature range), the increased molecular motion leads to the molecules moving further apart. As a result, the volume of the substance expands.

Answer:

because its molecules move further apart