Yeast Gas


CLUSTER: Chemical Reactions

TIME: 2 days; 30-45 minutes per day for setup, 10-15 minutes per day for observations



Yeast, Fungus


Students will learn that yeast are one-celled organisms that need sugar and water to grow. As yeast digest sugar, they give off carbon dioxide gas.

Background Information:

Yeast are one-celled organisms. They are a type of fungi, related to mushrooms, mold and mildew. In order to grow, yeast need glucose (C6H12O6) or sucrose (table sugar, C12H22O11), water (H2O) and warm temperatures. As yeast digest sugar, they turn into alcohol. In the process they give off carbon dioxide (CO2) gas. The alcohol dissolves in the water and the gas escapes into the air. Yeast is used to make bread. The carbon dioxide is what makes the bread light and fluffy.

The heat of the oven kills the yeast and burns off the alcohol. You can test for carbon dioxide with limewater. When you feed yeast in the presence of limewater, the calcium in the limewater reacts with carbon dioxide gas from the yeast to form calcium carbonate. Calcium carbonate stays in suspension and creates a milky colour. The chemical reaction is Ca(OH)2+CO2 CaCO3+H2O.

If too much carbon dioxide reacts with the limewater, then the calcium carbonate becomes calcium bicarbonate and dissolves. The solution becomes clear again. The chemical reaction is CaCO3+H2O+CO2 Ca(HCO3)2.

Activity Materials:

Experiment 1:
Fill a balloon with carbon dioxide.
Experiment 2:
The limewater test.
Each team will need: Each team will need:
  • A 4-litre jug
  • 250ml sugar
  • 1 package yeast
  • A large balloon
  • Water
  • 30ml molasses
  • Clean mixing spoon
  • 4-litre jars with wide mouths
  • 2 small glasses
  • 1 package yeast
  • Limewater(purchase from a chemical supply company, or make it yourself: mix lime, also know as calcium oxide, and water in a jar or breaker. Let the mixture settle, then pour the clear limewater off the top.)
  • 15ml molasses
  • Clean mixing spoon

Questions to Begin:

  1. What by-products do yeast give off when they digest sugar? (Here's a hint: think about rising bread dough)
  2. Can you think of an experiment that would test your idea?


Experiment 1:

  1. Split the class into teams. Each team will perform their own experiment.

  2. Distribute a 4-litre jug and a large balloon to each team. Have one student in each team inflate then deflate the balloon to loosen up the rubber.

  3. Have each team fill their jug 3/4 full of water, then mix 250 ml of sugar, 30ml of molasses, and a packet of yeast in the jug of water. Set in a warm place and attach the balloon to the mouth of the jug. Have the students record their hypothesis regarding what will happen. (The balloon should fill with carbon dioxide overnight or within 3 to 4 days.)

  4. Each day have students observe the balloons and record their observations.

  5. On day 3 to 4, have the students hold the opening of the inflated balloon to their mouths or noses and "taste" some of the gas. (It may smell like molasses, and will definitely have a tingle as from a soft drink.) Have students record their observations.
Experiment 2:

  1. You can set up this experiment as a demonstration or split the class into teams and have each team set up their own experiment.

  2. Fill a glass with limewater and carefully set it inside one of the gallon jars.

  3. Empty a packet of yeast into the jar, and add the molasses. Add water to the jar until it's about 2½ cm from the top of the glass. Be careful not to get yeast, molasses or water into the glass of limewater.

  4. Have students record their hypotheses about what will happen to the limewater.

  5. In a second 4-litre jug, place another glass of limewater. Fill the jug with water until it's about 2½ cm from the top of the glass. This will be the control.

  6. Leave both containers in a warm, dark place for two days. Be careful when moving the jars so that you don't spill the glass of limewater into the yeast solution. You may want to hold the glass in place.

  7. On day 3, have the students observe the jars and record their observations. (The limewater in the jug with yeast should turn milky white from carbon dioxide gas given off by the yeast, while the limewater in the control jar should stay clear.)

Question to Close:

  1. What caused the balloon to inflate? Where do you suppose the gas came from?
  2. Would the balloon fill if you didn't add the molasses to the solution?
  3. Why did the limewater turn colour but the control did not?
  4. Where did the carbon dioxide that turned the limewater colour come from?

Adapted from Brown, Robert J. 333 Science Tricks and Experiments. Blue Ridge Summit, PA: Tab Books, Inc., 1984. Brown, Robert J. 333 More Science Tricks and Experiments. Blue Ridge Summit, PA: Tab Books, 1984.

Additional Sources Bearman, Gerry. Ocean Chemistry and Deep-Sea Sediments. Oxford: Pergamon Press in association with The Open University, Milton Keyes, England,1989. Brady, James E. and Gerald E.Humiston. General Chemistry, Principles and Structures. 3rd edition. New York: John Wiley& Sons, 1982. Permission has also been granted courtesy of BBH Exhibits and Pfizer U.S.