Monday, June 28, 2010

Sublimation, Decanting and Centrifuging

4.1 Sublimation

Sublimation separates a mixture of solids, one of which sublimes. A few substances change directly, from a solid to vapour on heating without going through the liquid state. This change is called sublimation. On cooling, water vapour changes back to solid directly.
Iodine is a solid that sublimes. if a mixture of idone and sand is heated in a beaker, the iodine changes from solid to vapour directly. This is a picture of sublimation process.

The vapour changes back to solid directly on a cold surface. The sand is not affected by the heat and remains in the beaker. Other substances that sublime include carbon dioxide, anhydrous iron (III) chloride, anhydrous aluminium chloride and some ammounium compounds. As only a few solids sublime, this method of separation is limited.

4.2: Centrifuging/Decanting

These are used as alternative techniques to filtration. Decanting means carefully pouring off the liquid and leaving the undissolved solid, for example, sand, at the bottom of the beaker. It is quicker than filtration but separation may not be so effective.
Centrifuging is again quicker than filtration and is especially useful for separating fine insoluble solid particles from a liquid. As these fine particles are light, they are held in suspension throughout the water and only sink to the botttom very slowly. So decanting would be impossible. They are called suspensions. Some examples of suspensions are clay particles in water, chalk particles in water and blood cells in plasma. In the centrifuge, an electric motor causes the test tubes to revolve at high speed. This flings the solid to the bottom of the test tube where they collect. The clear liquid can then be decanted off. A suitable appartatus for decanting is shown here.