Agriculture and Food 7–12 · Years 11–12
Ethanol from sugar: following fermentation by mass loss
Agri-food, Fibre and Fuel Technologies (HSC elective)
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The idea
Yeast ferments glucose to ethanol and carbon dioxide in a fixed ratio, so the mass of carbon dioxide lost from a fermenting flask tells how much ethanol, a biofuel, has been made.
Safety card
Hazards
- limewater irritates skin and eyes
- warm water
- small amounts of ethanol vapour
Controls
- eye protection
- no distillation or burning of the product is needed
- nothing is drunk
Note
NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.
What you need
- glucose, 5 g per flask
- fast-acting dried yeast, 1 g per flask
- warm water at 30–40 °C, 50 mL per flask
- 100 mL conical flasks with loose cotton wool plugs, 4 (three with yeast, one control without yeast)
- balance reading to 0.01 g
- limewater, 5 mL in a boiling tube
- eye protection
How to do it
- Dissolve 5 g of glucose in 50 mL of warm water in each flask; add 1 g of yeast to three flasks and none to the control; plug each loosely with cotton wool.
- Record the mass of each flask at the start and then daily for 5 days (or every 30 minutes in the first lesson).
- Pour the gas from above the liquid of one yeast flask into the limewater (not the liquid) and note what happens.
- Subtract the control flask's mass loss (evaporation) from each yeast flask's loss to find the carbon dioxide lost.
- Convert carbon dioxide lost to ethanol formed with the model and express it as a percentage of the theoretical maximum from 5 g of glucose.
- Use the result to discuss grain and sugar as biofuel feedstocks against world food demand.
What you should see
The limewater turns milky and the liquid smells of ethanol. Net mass loss rises quickly and then levels off as the glucose is used up. From 5.00 g of glucose, C6H12O6 -> 2C2H5OH + 2CO2 allows at most 2.443 g of carbon dioxide and 2.557 g of ethanol; the measured loss is lower because some glucose feeds yeast growth and some carbon dioxide stays dissolved. The learner knows it worked when the control flask loses far less mass than the yeast flasks.
What changes
- What you change
- time (and yeast present or absent)
- What you measure
- net mass lost (g)
- What you keep the same
- 5 g glucose
- 1 g yeast
- 50 mL water at the same start temperature
- flask type and cotton wool plug
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Making fuel from grain has no effect on food supply.
- The mass lost during fermentation is ethanol evaporating.
- Yeast turns all of the sugar into ethanol.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
Sources
The pages the author read to write this activity.
- edu.rsc.org/experiments/fermentation-of-glucose-using-yeast-14-16-years/470.article
- edu.rsc.org/ideas/give-context-to-your-teaching-of-biofuels/4014389.article
- www.nsw.gov.au/education-and-training/nesa/curriculum/tas/agriculture-stage-6-2013
- www.nsw.gov.au/sites/default/files/noindex/2025-03/agriculture-amended-st6-syl-2013.doc
- curriculum.nsw.edu.au/learning-areas/tas/agriculture-11-12-2026/outcomes
- education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/5._Volume_2_Appendices.pdf