Explain why some rubber-like substances contract with rising temperature.

Explanation

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 vibration dominate over the transverse vibration. Such substances expand on heating. In some other crystals like rubber (sonsisting of long interwined and cross-linked chains of atoms in roughly randon orientation), transverse vibrations dominate over longitudinal vibration. Such solids contract on heating.