Usually when solids are heated they melt and becomes liquid. Then on further heating the liquid boils to form a gas. When we keep a piece of ice in a dish it melts and the water so formed evaporates after sometime. However there are some substances which on heating directly turn into gases from solids. camphor iodine crystals and solid carbondioxide are such substances. Such substances are called sublimatory substances and this process of conversion is known as sublimation. All substances are made up of atoms and molecules. These particles are held together by a force of attraction. If the force of attraction is weak the particles can be separated easily.In a block of ice the force of attraction is quite weak. With a small amount of heat the ice melts into water and further to form water vapours.
Similarly the camphor molecules are held together by weak force, even weaker than ice molecules and simply evaporate on heating without melting. A piece of camphor gives off a strong smell. It means that even at a moderate temperature the molecules of camphor escape and spread everywhere. The atmospheric heat is enough to separate its molecules. At 179c camphor melts to form a liquid but the moment it melts it catches fire and starts burning. However if we remove all oxygen from its surroundings then it melts. you burn camphor in an open dish you can see the burning piece of camphor floating on a colourless liquid. This is the liquid camphor. camphor is used in the manufacture of celluloid and also in many other industries.
