Usually when solids are heated they melt and become 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 carbon-dioxide are such substances. Such substances are called sublimatory substances and this process of conversion is known as sublimation. Do you know why does this happen?
As we know, 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 heat separates the particles of water further to form water vapours. Similarly the camphor molecules are held together by weak forces, 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 179°C 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. Now camphor can’t burn, because there will be no oxygen around it.
If 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.
