Aanandi took two test tubes and marked them A and B. She put two spoonfuls of sugar solution in each of the test tubes. In test tube B, she added a spoonful of yeast. Then she attached two incompletely inflated balloons to the mouth of each test tube. She kept the set-up in a warm place, away from sunlight.
What do you predict will happen after 3–4 days? She observed that the balloon attached to test tube B was inflated. What can be a possible explanation for this? (a) Water evaporated in test tube B and filled the balloon with the water vapour. (b) The warm atmosphere expanded the air inside the test tube B, which inflated the balloon. (c) Yeast produced a gas inside the test tube B which inflated the balloon. (d) Sugar reacted with warm air, which produced gas, eventually inflating the balloon.
C
She took another test tube, 1/4 filled with lime water. She removed the balloon from test tube B in such a manner that the gas inside the balloon did not escape. She attached the balloon to the test tube with lime water and shook it well. What do you think she wants to find out?
Aanandi wants to test whether the gas produced by yeast fermentation is carbon dioxide. When carbon dioxide gas is passed through lime water, it turns milky or cloudy, confirming the presence of CO₂.
Explanation
(i) Activity 2.8 in the chapter demonstrates that yeast is a microorganism belonging to the fungi group. When yeast is added to flour with sugar and warm water, it respires and breaks down food to release energy. During this process, carbon dioxide gas is released, which forms bubbles. Similarly, in test tube B, yeast ferments the sugar solution and releases carbon dioxide gas, which inflates the balloon. Test tube A without yeast shows no such inflation. (ii) The chapter explains in Activity 2.8 that yeast respires and breaks down sugar, releasing carbon dioxide during the process. Lime water (calcium hydroxide solution) is a standard test for carbon dioxide—it turns milky when CO₂ is passed through it due to the formation of calcium carbonate. This experiment confirms that the gas produced by yeast fermentation is indeed carbon dioxide.