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Question
Observe the set-up given in Fig. 2.15 and answer the following questions.
(i) What happens to the sugar solution in flask A?
(ii) What do you observe in test tube B after four hours? Why do you think this happened?
(iii) What would happen if yeast was not added in flask A?
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Answer
What happens in the flask A and test tube B set-up?
Warm sugar solution + yeast
In flask A
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Yeast ferments the sugar
It respires and breaks the sugar down
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Carbon dioxide is released
The gas travels along the tube into test tube B
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The lime water turns milky
Milky lime water is the test for carbon dioxide
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With no yeast in flask A, no gas forms and the lime water stays clear
(i) Yeast ferments the sugar. (ii) Lime water turns milky from carbon dioxide. (iii) Without yeast, nothing happens.
Explanation
- (i) Yeast feeds on the sugar solution and breaks it down (fermentation).
- (ii) It releases carbon dioxide, which turns the lime water in B milky.
- (iii) With no yeast, no gas is made, so the lime water stays clear.
Yeast ferments sugar to carbon dioxide, turning lime water milky; no yeast means no change.
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