A student is given glucose, dried yeast, warm water, limewater and the usual laboratory apparatus. The student wants to show that fermentation of glucose pr...

This question is about fermentation, an anaerobic process in which yeast converts glucose to ethanol and carbon dioxide: \( \text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 \). The design must let the gas out to be tested while keeping air (oxygen) out.

(a) A good method scores up to four marks [4]: dissolve the glucose in warm water and add the yeast in a conical flask [1]; fit a bung carrying a delivery tube that leads into a test tube of limewater [1]; keep the flask warm, about 30 to 40 °C, in a water bath so the yeast enzymes work quickly [1]; and the delivery tube (or an airlock) lets the gas bubble out but stops air getting back in, so the yeast respires without oxygen [1]. If oxygen entered, the ethanol would be oxidised to ethanoic acid (vinegar) and the culture would spoil.

(b) The gas bubbled through limewater turns it milky / cloudy [1]; the gas is carbon dioxide [1]. Limewater (calcium hydroxide solution) reacts with carbon dioxide to form insoluble calcium carbonate, which is the milkiness.

(c) Fermentation has stopped when the bubbling / fizzing stops, so no more gas passes into the limewater [1]. This happens when the glucose is used up, or when the ethanol concentration rises high enough to kill the yeast.