Question 13 of 14intermediate🔧 ApplyShort Answer5 marks

Yeast is added while baking bread to make it fluffy and soft. How does yeast work? Try and find out! Take a small bottle, some sugar, fresh yeast, water, and a balloon. Make a sugar solution in the bottle by mixing two teaspoons of sugar and a small amount of water. Now add a spoonful of yeast and cover the mouth of the bottle with a balloon. Leave it undisturbed for about an hour. What do you observe? Carefully take off the balloon, holding its mouth tightly closed and attach it to another small bottle containing freshly prepared lime water. Shake the bottle so that the contents of the balloon get mixed with lime water. What do you observe? What can you conclude from this experiment? Identify all the changes occurring in the experiment and state which of them are physical and chemical changes.

Correct Answer

Observations:

After about an hour, the balloon inflates or swells up due to gas produced by yeast.

When the gas from the balloon is mixed with lime water, the lime water turns milky.

Conclusions:

Yeast ferments sugar to produce carbon dioxide gas. The carbon dioxide gas inflates the balloon.

The turning of lime water milky confirms the presence of carbon dioxide.

Changes in the experiment:

  1. Dissolving sugar in water — Physical change (sugar dissolves but can be recovered by evaporation).

  2. Fermentation of sugar by yeast — Chemical change (new substances including carbon dioxide and alcohol are formed).

  3. Inflation of balloon — Physical change (only shape changes).

  4. Carbon dioxide turning lime water milky — Chemical change (calcium carbonate is formed).

In bread making, the carbon dioxide produced by yeast gets trapped in the dough, creating bubbles that make the bread fluffy and soft when baked.

Exercise: Exploratory Projects | Q: EP4 | (Page Page 16)
For More Understanding

Explanation

This activity demonstrates fermentation, a chemical change where yeast converts sugar into carbon dioxide and alcohol. The chapter explains that chemical changes involve formation of new substances. The lime water test confirms carbon dioxide production, similar to Activity 5.4 where carbon dioxide from vinegar and baking soda turned lime water milky.

Solution Steps

  1. Yeast ferments sugar to produce carbon dioxide gas

  2. Carbon dioxide gas inflates the balloon

  3. Lime water turns milky confirming presence of carbon dioxide

  4. Dissolving sugar in water is a physical change (sugar can be recovered)

  5. Fermentation of sugar by yeast is a chemical change (new substances formed)

  6. Inflation of balloon is a physical change (only shape changes)

  7. Carbon dioxide turning lime water milky is a chemical change (calcium carbonate formed)

  8. In bread making, carbon dioxide trapped in dough creates bubbles making bread fluffy and soft

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