Why is there a need to maintain the temperature reading of the mixture inside the distilling flask?
Answer:
1 ). Purified compounds will boil, and thus turn into vapors, over a relatively small temperature range (2 or 3oC); by carefully watching the temperature in the distillation flask, it is possible to affect a reasonably good separation.
2.). As distillation progresses, the concentration of the lowest boiling component will steadily decrease. Eventually the temperature within the apparatus will begin to change; a pure compound is no longer being distilled.
3 ). The temperature will continue to increase until the boiling point of the next-lowest-boiling compound is approached. When the temperature again stabilizes, another pure fraction of the distillate can be collected.
If there are several chemicals in the mixture you are distilling, then each one will have a certain temperature at which it will turn into a gas and move up into the distillation tubing. So if you're interested in only one of those chemicals, you need to know the temperature at which it will boil, evaporate and move into the tubing, and keep it at approximately that temperature for as long as you can until you can see there is no longer any condensation in the tubing at that temperature.
When a mixture of liquids with different boiling points is distilled, the heat added should not be too high as the heavier substances will vaporise along with the lightest causing a mixed vapour.
The temperature, taken at a point opposite to the vapour outlet to the condenser will be the boiling temperature of the lightest component and, with careful control of heat input, this temperature will not change until the next heaviest component vapour begins to increase this temperature.
At this point, the receiver should be changed to receive the 2nd component liquid as it condenses and, a little more heat may be added as long as the top temperature doesn't increase.
This process is repeated until the flask contains only an oily residue of heavy ends.
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