Project 4 - How Yeast Works in Bread Making

Last updated at July 13, 2026 by

  • Objective — See the chemical change that makes bread rise.
  • Materials — A small bottle, two teaspoons of sugar, fresh yeast, water, and a balloon.
  • Step 1 — Make a sugar solution in the bottle by dissolving sugar in water. Add a spoonful of yeast. Cover the mouth of the bottle with the balloon. Leave it undisturbed for about an hour.
  • Observation 1 — The balloon inflates on its own — yeast is producing a gas.
  • Step 2 — Carefully take off the balloon while pinching its mouth shut. Attach it to another small bottle containing freshly prepared lime water. Shake so the gas mixes with the lime water.
  • Observation 2 — The lime water turns milky — the gas is carbon dioxide.
  • Conclusion — Yeast feeds on sugar and produces carbon dioxide (and a small amount of alcohol). This gas makes bread rise during baking.
  • Identify the changes — Sugar dissolving (physical); yeast fermenting sugar (chemical); CO₂ turning lime water milky (chemical).
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