Effect of pressure on boiling points

The boiling points of liquids quoted in reference books refer to the values measured at a standard atmospheric pressure of 760 mm mercury. As we all know, atmospheric pressure changes, varying considerably from day-to-day as weather patterns change and cold or warm fronts cross the region. Atmospheric pressure also changes with elevation. Not everyone lives at sea level under a stable air pressure of 760 mm Hg so the following table will allow you to interpret any temperature readings you might get in terms of ambient atmospheric conditions.

Boiling point

Pressure Elevation Ethanol Water

Boiling point

Pressure Elevation Ethanol Water

psi

mm Hg

inches Hg

kPa

millibars

Feet

°C.

°C

16.5

853

33.6

113.7

1137

- 3280

81.5

103.3

15.6

806

31.8

107.5

1075

- 1640

79.9

101.7

14.7

760

29.9

101.3

1013

Sea level

78.4

100.0

13.9

716

28.2

95.4

954

1640

77.0

98.3

13.0

674

26.5

89.8

898

3281

75.6

96.7

12.3

634

25.0

84.5

845

4921

74.2

95.0

Not too many of us live below sea level but quite a few must live at elevations of several thousand feet, and it will be seen from the above table that the effect on the boiling point of ethanol is far from trivial. The same holds true of changes in atmospheric pressure at a fixed elevation, due in this case to the movement of air masses.

You will recall from the discussion of temperature changes during fractional distillation that, after the column has reached equilibrium, the heads are bled off until the temperature remains constant, indicating that pure ethanol is now distilling over. Clearly, to avoid being misled, it is useful to have some idea of what the boiling point of pure ethanol is on that particular day. The table will help in this regard.

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