EVAPORATION and VAPOUR PRESSURE PDF Print E-mail
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Tuesday, 16 March 2010 12:47
Article Index
EVAPORATION and VAPOUR PRESSURE
THE BOILING POINT OF A LIQUID
VAPOUR PRESSURE of LIQUID MIXTURES
DISTILLATION PROCESSES
BATCH DISTILLATION
FRACTIONAL DISTILLATION
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'Evaporation' is the process of changing liquid to vapour. This process can be used to separate a mixture of liquids which have different boiling point temperatures. It is known that a liquid produces vapour at any temperature.



This is due to the molecular motion which occurs at any temperature above absolute zero. The motion causes some of the molecules to 'jump out' of the liquid into the space above the liquid. The higher the temperature, the more molecules enter the vapour space.

The number of molecules in the vapour space cause a 'Vapour Pressure' above the liquid and, again, the higher the temperature, the greater the vapour pressure. The vapour pressure is referred to as the 'VP' of the liquid at a specific temperature. For any given liquid at a specific temperature, the number of molecules in the vapour space is constant, thereby giving a constant VP

If we consider a pure substance like water at a constant temperature, the motion of the molecules causes some to leave the liquid and enter the vapour space creating a vapour pressure. For example, the vapour pressure of water at 32 °F (0 °C) is 0.1 psia, well below atmospheric pressure. At 100 °F

(37.8 °C) the VP is 1.0 psia and at 212 °F (100 °C) it is 14.7 psia (101 kPa). The point at which a vapour pressure of a liquid is at its maximum for the specific temperature, the VP is called the Saturated Vapour Pressure (SVP) and, at this point the liquid and its vapour are said to be at 'Equilibrium'. (See Figure: 1)


Figure : 1

At equilibrium, the number of molecules entering the vapour space is equal to the number returning to the liquid. This condition remains constant as long as the temperature remains constant.

Increase or decrease in temperature will result in an increase or decrease in the number of molecules in the vapour phase and therefore an increase or decrease in the VP - but, when the temperature stabilises, the liquid and vapour will again be at equilibrium and show the VP for that temperature.



Last Updated on Tuesday, 16 March 2010 12:54