Yeast balloon experiment
01 // Question
How does water temperature change how much a balloon inflates when yeast, sugar and water react in a bottle?
A testable question names one thing you change (water temperature) and one thing you measure (balloon size).
02 // Variables
Independent
Water temperature- Cold water
- Warm water (~100–110°F)
- Very hot water (~150°F+)
Dependent
Balloon circumference after 20 minutes- Circumference in cm
- Qualitative rating of inflation (none, small, large)
03 // Hypothesis
IF yeast is mixed with sugar water at a warm, but not scalding, temperature, THEN the balloon will inflate the most compared to cold or very hot water, BECAUSE yeast are living microorganisms that feed and release carbon dioxide gas fastest within their ideal temperature range, and extreme heat kills them instead.
04 // Procedure
1. Measure the water
Measure equal amounts of water at each test temperature into separate bottles.
2. Add sugar and yeast
Stir in the same amount of sugar, then add the same amount of yeast to each bottle.
3. Attach the balloon
Stretch a balloon over the mouth of each bottle without letting the mixture spill.
4. Wait and observe
Let the bottles sit undisturbed for 20 minutes, checking periodically for foaming and balloon inflation.
5. Measure the balloon
At 20 minutes, measure each balloon's circumference with a string and ruler.
05 // Data table
| Water temperature | Foaming observed? | Balloon circumference at 20 min (cm) |
|---|---|---|
| Cold water | ____ | ____ |
| Warm water (~100–110°F) | ____ | ____ |
| Very hot water (~150°F+) | ____ | ____ |
06 // What to expect
The warm water bottle usually inflates the most, because yeast are living organisms that feed on sugar through a process related to fermentation, releasing carbon dioxide gas fastest at temperatures close to their ideal range.
The very hot water bottle often barely inflates at all, since heat that's too extreme kills the yeast before they can feed — while cold water simply slows the yeast down without killing them, so it inflates slowly rather than not at all.
07 // Lab notes
- Use a kitchen thermometer to check water temperatures rather than guessing — 'warm' and 'very hot' need to be reasonably consistent to compare trials fairly.
- Safety. Very hot tap or kettle water can scald — have an adult handle pouring water above about 120°F.
- Stretch the balloon on securely, or gas will leak out around its base instead of inflating it.
08 // Remember
Yeast are alive. Too cold and they're sluggish. Too hot and they're dead — warm is where they actually work.
09 // Questions people ask
Why does yeast make a balloon inflate?
Yeast are single-celled living organisms that feed on sugar and release carbon dioxide gas as a byproduct, in a process closely related to the fermentation used in bread-making and brewing. That released gas has nowhere to go but into the balloon stretched over the bottle, inflating it.
Why doesn't the balloon inflate much in very hot water?
Yeast are living cells, and extreme heat damages the proteins they need to function — similar to how boiling denatures an enzyme. Water that's too hot can kill the yeast almost immediately, so there's little to no gas production and the balloon barely inflates.
Why is sugar needed for the yeast balloon experiment?
Sugar is the yeast's food source. Without a sugar supply, the yeast have nothing to feed on and produce very little carbon dioxide gas, so the balloon stays mostly flat no matter what temperature the water is.