Boiling Point (Karachi Board, TextBook Board)




Ø The temperature at which vapour pressure of liquid is equal to the external pressure (atmospheric pressure) is called Boiling temperature or boiling point of that liquid.
Ø By increasing temperature, vapour pressure of liquid also increases.
Example: At 40°C the vapour pressure of water is 55.3 torr and at 95°C it is 634 torr.
Ø When the outside pressure is the 1 atmospheric pressure (760 torr), the boiling point is referred to as the normal boiling point.

Ø Boiling point of liquid varies with the outside pressure.
Example: In mountains regions, the boiling point of water will be bellow 100°C because the atmospheric pressure is below 760 torr.
Ø Some liquids decompose at their boiling point temperatures. Such a liquids are distilled at low temperature under reduced pressure.
Example: Glycerine boils at 290°C at 760 torr, at this temperature glycerine decomposes. So, under reduced pressure (50 torr), glycerine boils at 210°C and can easily be distilled without decomposition.

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