SUBLIMATION

 

(noun) When a substance has been transformed directly from the solid to the gaseous state without becoming a liquid.

(noun) A separation technique used to purify a solid chemical substance.

The substance to be purified is placed into a vacuum filter flask that has been fitted with a cold finger, and the flask is then placed onto an electric hotplate. Next, the water pump providing the vacuum is turned on to lower the pressure within the vacuum filter flask. The heat is then turned on to sublime the substance within the flask.

The hot chemical vapours rise until they hit the cold finger, where they condense. The condensed substance is called the sublimate and is a pure form of the substance that was sublimed.

Various other impurities initially present will either not sublime, and hence remain at the bottom of the filter flask, or they will not condense onto the cold finger and will be sucked out of the filter flask by the water pump. This general method is called distillation when applied to a mixture of liquids.