Bigger but Not Heavier? Following the Atoms in Bread Dough

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Bigger but Not Heavier? Following the Atoms in Bread Dough

Use kitchen data and an atom model to explain conservation of mass.

Weighing the dough

Read the passage and study Table 1. Then answer the questions.

Does Rising Dough Get Heavier?

1Rhea Kapoor, a seventh grader, bakes bread with her grandfather on Sunday mornings. While the dough rises, it often doubles in size. One Sunday, Rhea wondered whether a bigger lump of dough also has more mass, so she and her grandfather turned the next batch into an investigation.

2Dough rises because of yeast, a kind of tiny one-celled fungus. The yeast first breaks table sugar into simpler sugars such as glucose. Then it changes the glucose into new substances in a chemical reaction: C₆H₁₂O₆ → 2 C₂H₆O + 2 CO₂. The products are ethanol (C₂H₆O), a liquid, and carbon dioxide (CO₂), a gas. Bubbles of carbon dioxide get trapped in the stretchy dough and puff it up. For every 18.0 g of glucose that reacts, 9.2 g of ethanol forms.

3Rhea mixed two identical batches of dough, each made from 300 g of flour, 200 g of water, 10 g of sugar, 7 g of yeast and 6 g of salt. She put each batch in an identical large glass bowl. She covered Bowl A tightly with plastic wrap pressed down all around the rim, and she left Bowl B uncovered. The bowls were large enough that the dough never touched the wrap. Both bowls sat side by side on a warm counter. Every hour, Rhea weighed each bowl, with everything in it or on it, on the same kitchen scale, which reads to 0.1 g. She also measured how tall the dough in Bowl A was. Her results are in Table 1.

4After 2 hours, her grandfather shaped the dough from Bowl A into a loaf and baked it in a hot oven. In the oven, the gas bubbles grow, the ethanol and some of the water turn into gases, and the gases drift out of the loaf. Rhea weighed the loaf after it had cooled. The 523 g of dough had become a loaf with a mass of 460 g.

Table 1. Rising Dough on the Kitchen Scale
Time (h)	Bowl A, covered (g)	Bowl B, uncovered (g)	Dough height in A (cm)
0	1,428.0	1,423.0	6
1	1,428.0	1,422.1	9
2	1,428.0	1,421.2	12

1.

Use Table 1.

In 2 hours, the dough in Bowl A grew cm taller, and Bowl B lost g of mass.

2.

The dough in Bowl A doubled in height, but the mass of Bowl A did not change. Which explanation fits the data best?

A. The yeast made new atoms as it grew, but the plastic wrap pressed down and hid their mass.B. Gas has no mass, so the bubbles made the dough bigger without adding any grams.C. The dough lost some mass as it rose, and the plastic wrap gained exactly the same amount.D. The gas in the bubbles was made from atoms already in the bowl, and none of them could leave.

3.

Why did Bowl B lose mass while Bowl A did not?

A. Some carbon dioxide and water vapor escaped from the open bowl into the kitchen air.B. The yeast in the open bowl destroyed some of the sugar atoms as it made gas bubbles.C. The open bowl let in cool air, which made the dough shrink and weigh a little less.D. The dough in the open bowl rose higher, so less of it pressed down on the scale.

4.

Use paragraph 4.

During baking and cooling, the dough lost g of mass.

5.

Rhea's grandfather jokes that the oven "burns away" part of the dough. Which statement best describes what happened to the missing mass?

A. The heat turned some of the dough's mass into the energy that browned the crust.B. Its atoms left the loaf in gases such as water vapor, ethanol and carbon dioxide.C. The loaf's atoms got lighter as they warmed up and then stayed light as it cooled.D. The oven's heat destroyed the atoms in some of the water and ethanol.

Modeling the yeast reaction

Use the reaction in paragraph 2: C₆H₁₂O₆ → 2 C₂H₆O + 2 CO₂.

6.

Rhea builds the reaction from clay balls. Her first model of the products is 2 ethanol molecules (C₂H₆O) and 1 carbon dioxide molecule (CO₂). What is wrong with this model?

A. It has the right atoms, but the ethanol and CO₂ should be joined into one molecule.B. It has too few atoms, but that is fine because the gas floats away from the dough.C. It has 1 carbon and 2 oxygen atoms fewer than the glucose, so a CO₂ is missing.D. It has too many hydrogen atoms, so 1 ethanol molecule should be taken away.

7.

Suppose 5 glucose molecules react.

They form carbon dioxide molecules, and the ethanol molecules that form hold hydrogen atoms in all.

8.

Use paragraph 2. Suppose 18.0 g of glucose reacts completely and all of the products are kept.

The carbon dioxide that forms has a mass of g.

9.

Use the reaction. Where do the carbon atoms in the carbon dioxide bubbles come from?

A. From the glucose molecules, which are rearranged by the reactionB. From the air in the kitchen, which the dough soaks up as it risesC. From the salt, which breaks apart when it touches the yeastD. From new carbon atoms that the yeast cells make as they grow

10.

Rhea's brother says, “The dough got bigger, so the yeast made new matter. Then the loaf got lighter, so the oven destroyed matter.” Write a claim, use evidence from Table 1 and paragraph 4, and use the atom model of the reaction to explain your reasoning.