Programme For International Student Assessment Pisa / Sample Science Passage 12: Bread Dough


4 To make bread dough, a cook mixes flour, water, salt and yeast. After mixing, the dough is placed in a  container for several hours to allow the process of fermentation to take place. During fermentation, a  chemical change occurs in the dough: the yeast (a single-celled fungus) helps to transform the starch and  sugars in the flour into carbon dioxide and alcohol. 

Four experiments




A few hours after mixing the dough, the cook weighs the dough and observes that its weight has decreased. The weight of the dough is the same at the start of each of the four experiments shown in the graph above. Which two experiments should the cook compare to test if the yeast is the cause of the loss of weight?

The cook should compare experiments 3 and 4.
The cook should compare experiments 2 and 4.
The cook should compare experiments 1 and 2.
The cook should compare experiments 1 and 3.

When the risen (leavened) dough is placed in the oven to bake, pockets of gas and vapours in the dough expand. Why do the gas and vapours expand when heated?

Their molecules move faster.
Their molecules increase in number.
Their molecules collide less frequently.
Their molecules get bigger.

Fermentation causes the dough to rise. Why does the dough rise?

The dough rises because a gas, carbon dioxide, is produced.
The dough rises because alcohol is produced and turns into a gas.
The dough rises because of single-celled fungi reproducing in it.
The dough rises because fermentation turns water into a vapour.

 
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