Fermentation of glucose by yeast: tracking carbon dioxide loss on a balance and testing for ethanol

The idea

Yeast enzymes convert glucose to ethanol and carbon dioxide; the mass lost from an open flask over days is the carbon dioxide, and the stoichiometry sets the maximum yield.

What you need

• Glucose 5.0 g, dried yeast 1.0 g, warm water (30 to 40 degrees Celsius) 50 mL, 100 mL conical flask; a second flask with yeast and water only as the control
• Cotton wool plug or an airlock (a delivery tube into limewater), balance to 0.01 g
• Limewater 5 mL, thermometer, incubator or warm cupboard at 30 degrees Celsius
• Distillation set (teacher) for the ethanol test: round-bottom flask, still head with thermometer, condenser and receiver; acidified permanganate for confirming ethanol

How to do it

1. Dissolve the glucose in the warm water, add the yeast, swirl and plug with cotton wool; weigh the whole flask and record the time.
2. Bubble the gas through limewater by fitting the delivery tube for the first hour and record the change.
3. Keep the flask warm and reweigh at the same time each day for four to five days, swirling before each reading; plot mass lost against time.
4. When the mass stops falling, note the smell; the teacher filters the class's flasks into one distillation flask, distils the pooled liquid and collects the first 5 mL of distillate by volume. With only about 5 percent ethanol in the liquor the still head reads close to 95 degrees Celsius from the start, not the 78 degrees Celsius of pure ethanol, so no temperature window is used. Test a few drops of the distillate with warm acidified permanganate and note its smell.
5. Calculate the maximum carbon dioxide mass and ethanol mass from the glucose used, the percentage of the theoretical carbon dioxide lost, and the ethanol concentration expected in the flask.
6. Compare the measured mass-loss curve with the model curve and account for the plateau.

What you should see

Limewater turns milky within the first hour. The flask loses mass steadily over the first days and then levels off; 5.00 g of glucose (27.8 mmol) can give at most 2.44 g of carbon dioxide (1.38 L at 25 degrees Celsius) and 2.56 g of ethanol (about 4.9 percent of the 52.6 g of solution left once the carbon dioxide has gone, yeast excluded); the measured loss falls short of that maximum because some carbon dioxide stays dissolved and some sugar goes to yeast growth, while a water-and-yeast control flask weighed alongside shows how much of the loss is evaporation through the plug. The still head reads close to 95 degrees Celsius, not 78 degrees Celsius, because a liquor of about 5 percent ethanol boils near 95 degrees Celsius and its vapour is only about 18 mole percent (36 percent by mass) ethanol; the first 5 mL of distillate smells of ethanol and decolourises warm acidified permanganate, but it is too dilute to be sure of burning, so no flame test is made.

What changes

What you change

time (and, as an extension, temperature or glucose mass)

What you measure

mass of carbon dioxide lost (g)

What you keep the same

• glucose and yeast masses
• water volume
• temperature
• plug type

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

• The mass loss is water evaporating (the control flask of water and yeast without glucose, weighed alongside, loses far less over the same days).
• Yeast eats the glucose and grows, so no chemical equation applies (the enzymes catalyse a fixed stoichiometry; only a small fraction of sugar goes to growth).
• Fermentation can make any strength of alcohol (rising ethanol concentration inhibits and finally stops the yeast, which is why stronger spirits are distilled).

Safety card

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

• Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-14CH11/12-5CH11/12-6
• Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 7, which it allows in any moduleCH11/12-3
• Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-03CH-12WS-03CH-12WS-05
• Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/chemistry-stage-6-2017
2. www.nsw.gov.au/sites/default/files/noindex/2025-03/chemistry-stage6-syllabus-word.docx
3. curriculum.nsw.edu.au/learning-areas/science/chemistry-11-12-2025/outcomes
4. edu.rsc.org/experiments/fermentation-of-glucose-using-yeast-14-16-years/470.article
5. education.nsw.gov.au/teaching-and-learning/curriculum/science/science-curriculum-resources-k-12/science-11-12-curriculum-resources/chemistry-year-11-and-12