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Science and Technology K–6 · Year 3

Is yeast alive? Sugar, warm water and a balloon

Science understanding: Biological sciences

PracticalLow risk

The idea

A living thing takes in food and gives off waste, so a packet of dry yeast that inflates a balloon when fed sugar shows a characteristic of life that flour does not.

What you need

  • 2 clean 500 mL PET bottles
  • 2 round balloons (25 cm)
  • 1 sachet dried baker's yeast, 7 g
  • 7 g plain flour (the non-living comparison)
  • 2 teaspoons white sugar (about 4 g each)
  • 500 mL warm tap water at 35 to 40 degrees Celsius, checked with a thermometer
  • tape measure or string and ruler
  • clock or timer

How to do it

  1. Put 250 mL of warm water (35 to 40 degrees Celsius) into each bottle.
  2. Add 4 g of sugar to each bottle and swirl until dissolved.
  3. Add the 7 g of yeast to bottle A and the 7 g of flour to bottle B. Swirl both for ten seconds.
  4. Stretch a balloon over the neck of each bottle straight away and start the timer.
  5. Every five minutes for 30 minutes, measure the widest circumference of each balloon with the tape and record it in a table.
  6. Record what you see and smell in each bottle (foam, bubbles, a bread smell).
  7. Repeat with a second pair of bottles on another day and compare the two runs.

What you should see

Bottle A foams and its balloon begins to inflate during the 30 minutes; bottle B stays flat. The gas is carbon dioxide from the yeast using the sugar. Upper bound computed from the sugar supplied: 4.0 g sucrose is 0.0117 mol, hydrolysed to 0.0234 mol hexose, which ferments to at most 0.0468 mol carbon dioxide, 1.18 L at 35 degrees Celsius and 101.3 kPa, while a balloon 25 cm across holds about 8.2 L, so even if all the sugar were used the balloon could fill only part-way; a balloon circumference that grows steadily while the flour balloon does not is the evidence.

What changes

What you change
whether the powder is yeast or flour
What you measure
balloon circumference after 30 minutes
What you keep the same
  • water volume
  • water temperature
  • sugar mass
  • bottle and balloon size
  • time

Common misconceptions

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

  • Yeast is a powder, so it cannot be alive.
  • The balloon inflates because the water is warm, not because of the yeast (the flour bottle at the same temperature shows otherwise).

Safety card

Low riskLearners carry it out

Hazards

  • warm water
  • yeast is a food allergen for a few learners
  • latex allergy (balloons are natural rubber latex)

Controls

  • water below 40 degrees Celsius
  • no tasting
  • wash hands after handling
  • check allergy records; a learner with a latex allergy does not handle the balloons, and the teacher fits them

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

Curriculum references

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

  • Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST2-4LW-SST2-1WS-S
  • Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST2-SCI-01
  • Australian Curriculum v9AC9S3U01AC9S3I03AC9S3I05

Sources

The pages the author read to write this activity.

  1. www.scootle.edu.au/ec/search?accContentId=AC9S3U01
  2. curriculum.nsw.edu.au/learning-areas/science/science-and-technology-k-6-2024/outcomes
  3. www.nsw.gov.au/education-and-training/nesa/curriculum/science/science-and-technology-k-6-2017
  4. primaryconnections.org.au/v84-sequences/marvellous-micro-organisms
  5. primaryconnections.org.au/v84-sequences/feathers-fur-or-leaves

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