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Yeast-Inflated Balloons

Students use yeast to explore CO2 production by living organisms.

This is an excellent opportunity for students to design their own experiments to determine which variables affect the yeast’s ability to produce CO2.

Yeast is a fungal microorganism that feeds on sugar and produces carbon dioxide (CO2) plus ethanol. As the yeast feeds on the sugar, it produces carbon dioxide gas. This process is known as fermentation. The trapped CO2 accumulates inside the balloon, slowly inflating it.

A very similar process happens as bread rises. Carbon dioxide from yeast fills thousands of balloon-like bubbles in the dough. This is what gives baked bread its airy texture.

Since yeast also produces alcohol as it feeds, it is an important ingredient in beer & wine production.


  • Determine variables used in an experiment of their own design.

  • Create a hypothesis

  • Describe one of the by products of respiration.

  • Describe the properties of gases.


  • Per Pair of Students:
    1 tbsp (15 ml) active dry yeast (not fast-acting)
    1 teaspoon (5 ml) sugar
    1 cup (250 ml) very warm water (41–46°C or 105–115°F )
    measuring tape (flexible kind)
    spot near a heat source (like a radiator or a sunny window)

Key Questions

  • What special characteristic of yeast made the balloon inflate?
  • Why was the sugar added?
  • Why did we need to put the balloon in a warm place?
  • Would you get the same results if the balloon was untied?

What To Do

  1. Measure the length and circumference of your balloon. Record the results.
  2. Place the small end of the funnel into the opening of the balloon.
  3. Pour 1 tablespoon of yeast and 1 teaspoon of sugar into the balloon using the funnel.
  4. Slowly add the cup of very warm water.
  5. Remove the funnel from the balloon and tie it closed.
  6. Place the balloon in a warm place.
  7. Measure the length and circumference of the balloon every 15 minutes for an hour. Record the results.

​Teacher Tip: Try fast-acting yeast if you need the yeast to work within a shorter period of time.


  • Design an experiment to explore one of the following questions:
  • Which sugar/food combination helps the yeast produce the most gas?
  • Try different foods for the yeast to ferment, e.g. brown sugar, syrup, honey, candy, salt. At what temperature is the yeast most active? At what temperatures is it unable to blow up the balloon?
  • Try varying the water temperature, using a thermometer to measure the temperature of the water.

Other Resources

Science World Resources | Ballon gonflé par les levures | French version of this resource