Yeast, sugar and a balloon with a hole
A learner tests how sugar concentration affects gas production by yeast. Five flasks each contain 10 g of yeast and 50 cm³ of water at 35 °C, plus sugar: 0 g, 2 g, 4 g, 6 g and 8 g. Each flask is sealed with a balloon and left for 20 minutes. Balloon circumference is measured with string and a ruler. After 8 minutes the 8 g balloon stops growing, and the learner finds a small hole in it. The room cools from 35 °C to 27 °C during the 20 minutes. Circumference results: 0 g = 6.0 cm, 2 g = 8.5 cm, 4 g = 11.0 cm, 6 g = 13.5 cm, 8 g = 9.0 cm.
1. The flask with no sugar still produced a balloon of 6.0 cm circumference. Explain this and show what it means for reading the results.
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With no sugar there is no substrate for fermentation, so this yeast cannot produce carbon dioxide by respiration. The 6.0 cm must therefore come from something else: air already in the flask expanding when it was warmed to 35 °C, plus the fact that a balloon starts with a non-zero size and some slack.
That makes 6.0 cm a baseline, not a zero. Subtracting it gives 2.5, 5.0 and 7.5 cm of extra inflation for 2, 4 and 6 g of sugar. The corrected figures rise steadily up to 6 g, which is the real pattern.
Reporting raw circumferences would overstate gas production at every concentration and would make the 0 g control look as though it were respiring. The control is doing exactly its job: it measures the size of the effect that is not due to sugar.
2. Should the 8 g result be used? What should the learner do next?
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No. The hole means gas escaped, so 9.0 cm underestimates the true inflation for 8 g. That reading measures a leak, not the yeast's activity, and it cannot be used to claim that gas production falls at high sugar concentration.
The valid points (0, 2, 4 and 6 g) show a roughly straight-line increase of about 1.25 cm per gram of sugar, so the defensible provisional statement is that gas production increases with sugar concentration over the range 0–6 g, with 8 g untested.
The learner should repeat the 8 g flask with a new balloon, ideally two or three times, and only then comment on whether the trend levels off. Simply deleting the point and drawing a straight line through the rest would hide a possible real effect.
3. The room cooled from 35 °C to 27 °C. Explain how this affects the results and how the method should be changed.
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Two effects act in the same direction. Yeast enzymes work more slowly at 27 °C than at 35 °C, so less carbon dioxide is made. And gas already in the balloon contracts as it cools, so the circumference shrinks even if the amount of gas is unchanged.
Both make later readings too small, and because flasks are likely to have been read one after another, the last flasks read may be the coolest. The comparison between sugar concentrations is therefore distorted, not merely noisy.
The fix is to stand all flasks in a water bath at 35 °C, to read every balloon at the same moment, and to record the temperature at each reading. Better still, replace the balloon with a gas syringe, which measures volume directly instead of relying on a stretched rubber membrane whose stiffness also changes with temperature.