A series of experiments are set up in flasks labelled A, B, C and D. Each flask contains 200 cm³ of water, 10 g of sugar, and 5 g of yeast. Flask A contains 200 cm³ of boiled water at 37 °C. Flask B c

Here's the solution to question 2:

a) State what could be observed in each of the flasks labelled A, B, C and D.
A. The balloon will not inflate; no bubbles will be observed in the mixture.
B. The balloon will not inflate; no bubbles will be observed in the mixture.
C. The balloon will inflate significantly; many bubbles will be observed in the mixture.
D. The balloon will not inflate or inflate very slightly; very few or no bubbles will be observed.

b) Explain briefly the observations made in each of the flasks labelled B, C and D.
B. The yeast was killed by the boiled water, so no fermentation occurred to produce gas.
C. The yeast was active at warm temperature and fermented the sugar, producing carbon dioxide gas which inflated the balloon.
D. The ice-cold water inhibited the activity of the yeast, significantly slowing down or stopping fermentation and gas production.

c) Suggest the content of the balloon fixed to the flask labelled C.
Carbon dioxide gas

d) Describe briefly a test that could be performed to confirm the content suggested in (c).
Bubble the gas through limewater (calcium hydroxide solution). If the gas is carbon dioxide, the limewater will turn cloudy or milky.

e) Identify the control experiment in the set-up.
Flask A (or Flask B)