Physics

Sound

Sublimation

Most of the solid substances, when heated, first change into a liquid and then into vapours (or gas).This vapour, on cooling, first forms a liquid and then a solid. But there are a few solid substances which change directly into vapours (or gas) on heating and the vapours give back solid substance on cooling (without passing through the liquid state). The changing of a solid directly into vapours on heating, of vapours into solid on cooling, is known as sublimation. Sublimation can be represented as :
Please note that:
(i) the changing of a solid directly into vapour (or gas) is called sublimation, and
(ii) the changing of vapour (or gas) directly into solid is also called sublimation.
The solid substance which undergoes sublimation is said to 'sublime'. The solid obtained by cooling the vapours of the solid is called a 'sublimate'. We will now give an example of sublimation.
Ammonium chloride undergoes sublimation. When solid ammonium chloride is heated, it directly changes into ammonium chloride vapour (without passing through liquid state). And when hot ammonium chloride vapour is cooled, it directly changes into solid ammonium chloride (without passing through the liquid state). Sublimation occurs in only a few substances. So, it is a rare process. The common substances which undergo sublimation are : Ammonium chloride, Iodine, Camphor, Naphthalene and Anthracene.When these solids are heated, their particles move so quickly that they separate completely to form vapour (or gas). And when these vapour (or gas) is cooled, these particles slow down so quickly that they become fixed and form a solid.Another example of sublimation is provided by solid carbon dioxide (which is commonly known as dry ice). Solid carbon dioxide (or dry ice) sublimes to form carbon dioxide gas. We will now perform an experiment to demonstrate the sublimation of ammonium chloride.
We take some ammonium chloride in a china dish and place the china dish on a tripod stand (see Figure 46). The china dish is covered with an inverted glass funnel. A loose cotton plug is put in the upper, open end of the funnel to prevent the ammonium chloride vapours from escaping into the atmosphere.The china dish is heated by using a burner. On heating, ammonium chloride changes into white vapours. These vapours rise up and get converted into solid ammonium chloride on coming in contact with the cold, inner walls of the funnel (see Figure 46). In this way, ammonium chloride collects on the inner sides of the funnel in the form of a sublimate and can be removed.
In many households, small naphthalene balls are kept in stored woollen and silk clothes, etc., to protect them from the attack of moths and other insects. It is noticed that these naphthalene balls disappear with time without leaving behind any residue.This is because naphthalene balls undergo sublimation. The naphthalene balls keep on forming naphthalene vapours slowly which disappear into the air. We can now show the interconversion between the three states of matter : solid, liquid and gas, with the help of 'states of matter triangle'shown in Figure 48.

Notes

Sound - Notes

1. 3. Humidity of Air