Answer is: density of the substance, energy of the substance, intermolecular bonds between particles.
Melting is a physical change (process), the same substance is present before and after the change, so chemical bonds and melting point of the substance are not changed.
Density of liquids are usually less than in solids, because atoms are further apart.
Liquids have higher energy and atoms or molecules move faster.
Intermolecular bonds are weaker in liquids.
