Question

Explain why the boiling temperature of an ideal mixture of two liquids increases as distillation proceeds. What changes about the mixture as distillation proceeds?

Answer 1

The normal boiling point (Kelvin or Celsius) (also called the atmospheric boiling point or the atmospheric pressure boiling point) of a liquid is the special case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level.

If a mixture of two miscible liquids with different boiling points is heated to boiling, the vapour will not have the same composition as the liquid; it will be richer in the more volatile component. Any mixture with a composition exactly equal to that of aseotropic will distill at a single constant temperature, just as if it were a pure liquid.

Some liquid mixtures, instead of forming ideal solutions with liquid­vapour composition , form either minimum or maximum boiling mixtures because of very different adhesive and cohesive forces. Ex, Ethanol and water form a constant boiling mixture (95.6% ethanol, 4.4% water) with a minimum boiling point of 78.2^o C (lower than that of pure ethanol, 78.3^o C, or pure water, 100^o C).

for mixture proceeds,

1.	Place 50 mL of the unknown mixture in the round bottomed distillation flask. Add several boiling chips. Check that all your connections are tight.
2.	Heat the mixture slowly and carefully.    Once the temperature starts to rise start recording the temperature every 30 seconds until heating is discontinued.
	Note: The leveling off of temperature indicates that one of the components of the mixture is boiling. Liquid will begin to collect in the collection flask. Remember that the thermometer is measuring the temperature of the vapors not the temperature of the liquid in the flask.
3.	Once the temperature has begun to rise again, immediately change the collection flask to collect the next component that distills. Carefully stopper your first flask and set it aside away from the bunsen flame.
4.	The temperature will again level off. Collect your second distillate and stopper it once the temperature starts to rise again.