Q: Why does some rubber like substances contract with rising temperature ?
Sol: Atoms in a solid vibrate longitudinally as well as transversely .The average separation of the planes of atoms increases due to their longitudinal vibrations and decreases due to their transverse vibrations with rise in temperature . Expansion depends on the dominance of one type of vibration over the other . In some crystals , the longitudinal vibrations dominate over the transverse vibrations . Such substances expand on heating . In some other crystals like rubber (consisting of long , intertwined and cross-linked chains of atoms in roughly random orientation ) , transverse vibrations dominate over longitudinal vibrations . Such solids contract on heating .