If the boiling points of two compounds differ by fifty degrees c at atmospheric pressure what would be the effect of dropping the pressure to ten mm?

Answer 1

Any time you drop the external temperature, it is going to make it easier for compounds to boil since the atmosphere is no longer exerting as much force down on the liquid. However, whether one or both compounds boils at that temperature depends less on the difference between their two boiling points and more on what their vapor pressure is at whatever temperature they are at.

Compounds boil when their vapor pressure (the pressure of the molecules that naturally escape the solid or liquid because they have more energy) is the same as the external pressure. The vapor pressure of a substance depends only on temperature, which is why the point at which something boils is tied to temperature.

Take water for instance. Water boils at 100 degrees C at sea level because at that temperature its vapor pressure is 1 ATM (760 mm Hg). At room temperature (25 degrees C) water has a vapor pressure of around 22 mm Hg. Since its vapor pressure at 25 degrees C exceeds 10 mm Hg, it would indeed boil at that pressure.

Now if a liquid had a boiling point that was 50 degrees below water's, it would presumably boil at that temperature as well (although not necessarily... depends on other factors). If a liquid had a boiling point that was 50 degrees higher than water, then there is a decent chance it may not boil, but you would really need info on its vapor pressure at room temperature to know.

Answer 2

Lowering the pressure to 10 mmHg would further decrease the boiling points of the compounds. The compound with the higher boiling point difference at atmospheric pressure would experience a greater decrease in boiling point compared to the other compound due to its higher initial boiling point, ultimately increasing the gap between their boiling points.