🧪 Ages 6–14⏱ 15 min + 30–60 min waiting🧽 Mess: Low
The question: Yeast is alive! What temperature helps it make the most gas?
Make a prediction
Which balloon will grow the biggest: cold, warm or hot water? Write your guess on the worksheet before you start. Scientists call this a hypothesis.
What you need
3 clean plastic bottles
3 balloons
3 packets (or 6 3/4 tsp) active dry yeast
3 tbsp sugar
Cold water, warm water (about 105–110°F, like a warm bath) and hot water (about 150°F)
A thermometer (optional)
A funnel
Steps
Label the bottles Cold, Warm and Hot.
Put 1 packet yeast and 1 tbsp sugar in each bottle.
Add 1 cup of the matching water to each bottle (an adult handles the hot water) and swirl gently.
Quickly stretch a balloon over each bottle opening.
Every 10 minutes, measure around each balloon with a string or tape measure and write it down.
🔬 Try a bottle with no sugar. Does the yeast still make gas?
⚠️ Safety: An adult should handle hot water.
What's happening? The science
Yeast is a tiny living fungus. When it is warm and has food (sugar), it eats the sugar and gives off carbon dioxide gas. This is called fermentation. The gas fills the balloon, and in bread dough it makes the dough rise.
Cold water keeps yeast sleepy and slow. Warm water (about 105–110°F) wakes it up. Water that is too hot can kill yeast, so it stops making gas.
Think further
Why do bread recipes say to use warm, not hot, water?
What do you think happens to the yeast when bread bakes in the oven?
