A student fermented a glucose and yeast mixture in the apparatus shown in Fig. 12.1, which uses an airlock. The airlock lets gas escape but stops air enteri...
Question 1 Report
A student fermented a glucose and yeast mixture in the apparatus shown in Fig. 12.1, which uses an airlock. The airlock lets gas escape but stops air entering the flask. The student recorded observations over 8 days in Table 12.1, then separated the ethanol from the mixture.
Table 12.1
| Day | Observation at the airlock F |
|---|---|
| 1 | no bubbles |
| 2 | steady stream of bubbles |
| 5 | ? |
| 8 | no bubbles |
(a) Name the parts labelled D, E and F on Fig. 12.1. [3]
(b) Complete Table 12.1 with a likely observation at the airlock on day 5, and explain your answer. [2]
(c) Explain why the airlock is needed for fermentation to work. [2]
(d) After 8 days the student separated the ethanol from the mixture. Describe how the ethanol could be obtained from the fermented mixture and how the student would know when to collect it. [5]
(e) The two liquids in the mixture are ethanol (boiling point 78 °C) and water (boiling point 100 °C). State which liquid is collected first and give a reason. [2]
(f) Complete the observation table used during the separation by giving the two column headings the student should record. [2]
Answer Details
This question follows a full fermentation then a distillation. Fermentation is anaerobic: \( \text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 \). Distillation then separates the ethanol from the water using their different boiling points.
(a) D is the conical flask (holding the glucose and yeast) [1]; E is the delivery tube (bent tube) [1]; F is the airlock (water trap) [1].
(b) On day 5 a likely observation is bubbles slowing down / fewer bubbles [1], because the glucose is being used up so fermentation is slowing and ethanol is building up [1]. (Steady bubbles with a supporting reason is also credited.)
(c) The airlock is needed because it lets the carbon dioxide gas escape so pressure does not build up in the flask [1], while keeping oxygen (air) out so the yeast respires anaerobically to make ethanol rather than the ethanol being oxidised to vinegar [1].
(d) To obtain the ethanol, use distillation [5]: heat the fermented mixture [1]; the ethanol has the lower boiling point so it boils off first, at about 78 °C [1]; pass the vapour through a condenser where it is cooled and condensed back to liquid [1]; collect the liquid while the thermometer reads about 78 °C [1]; the water, with the higher boiling point of 100 °C, stays behind in the flask [1]. The thermometer is placed at the mouth of the flask so it reads the temperature of the vapour actually passing over.
(e) Ethanol is collected first [1], because it has the lower boiling point (78 °C against 100 °C) so it vaporises first [1].
(f) The two column headings for the separation are the temperature on the thermometer / °C [1] and the observation (liquid collected) [1], so you can link what is collected to the temperature at which it distils over.
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