After fermentation was complete, a student heated the fermented mixture in the apparatus shown in Fig. 3.1 to separate the ethanol from the water. Ethanol b...
Question 1 Report
After fermentation was complete, a student heated the fermented mixture in the apparatus shown in Fig. 3.1 to separate the ethanol from the water. Ethanol boils at 78 °C and water boils at 100 °C.
(a) Name the parts of the apparatus labelled X and Y on Fig. 3.1. [2]
(b) State the temperature shown on X when liquid Z is being collected. [1]
(c) Name liquid Z. [1]
(d) Explain why the bulb of X is placed level with the side arm of the flask. [1]
(e) State the purpose of the cold water passing through the outer part of Y. [1]
Answer Details
This tests separating ethanol from water by fractional distillation, using the difference in their boiling points (ethanol \(78\ ^{\circ}\text{C}\), water \(100\ ^{\circ}\text{C}\)). The lower-boiling liquid distils over first.
(a) The labelled parts are: X = thermometer [1] and Y = (Liebig) condenser. [1]
(b) When liquid Z is being collected, X shows \(78\ ^{\circ}\text{C}\) (accept \(76\) to \(80\ ^{\circ}\text{C}\)). [1]
(c) Liquid Z is ethanol, [1] because it boils at the lower temperature and distils over before the water.
(d) The bulb of X is placed level with the side arm of the flask so that it measures the temperature of the vapour passing into the condenser, that is the boiling point of the substance actually being collected, not the liquid in the flask. [1]
(e) The cold water passing through the outer jacket of Y is there to cool and condense the vapour back to a liquid so it can be collected. [1]
Exam tip: in fractional distillation the reading stays near the boiling point of the substance coming over; a rise towards \(100\ ^{\circ}\text{C}\) warns that water is beginning to distil.
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