Changes state 4. Brings about chemical action. 5. Changes physical properties. Illustrations: (i) On a hot day while cycling on a road if we touch the handle of a bicycle instead of the grips, the handle will seem to be hotter than the grips because the handle is a good conductor of heat and when we touch any part of it, heat flows to the hand, not only from that part but also from the neigbouring parts: whereas in the case of grips the heat flows to the hand from only that part which is touched. The handle seems hotter because it is a good conductor of heat and the flow of heat to the hand is greater and more rapid than in the case of the grips. (ii) The rails on a railway line are laid with a small gap between them so that with a rise in temperature in summer the gap would provide room for expansion. Heat will also be generated when the train passes over the rails.
(iii) The iron tyre of a cart wheel is always made a bit smaller in diameter than the wooden wheel. After making the tyre red hot, it is slipped on the wheel, and water is poured on it. On cooling the iron tyre contracts, and holds the parts firmly together. (iv) To loosen a glass stopper which has struck fast in the neck of a glass bottle we heat the neck a little by turning it round over a flame.
Owing to the expansion of the neck the stopper can be easily pulled out. (v) Two plates which are to be fastened together are held fast, whilst a hole is drilled through them both. A red hot rivet is passed through the hole and is hammered until both ends have heads closely gripping the plates. The contraction of the rivet as it cools binds the plates together with a great force.