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Heavier With Rust? Following the Atoms From a Throwing Shed

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Heavier With Rust? Following the Atoms From a Throwing Shed

Use steel wool data and an atom model to explain why rusting iron changes mass.

The steel wool test

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

Rust in the Throwing Shed

1The track and field team at Brookhaven Middle School keeps its throwing gear in a shed beside the field. After a wet spring, Coach Gretchen Albrecht found orange-brown rust on several iron shot puts. “Rust eats the iron away,” she said, so she expected the rusty shot puts to be lighter than new ones. Tobias Nakamura-Reyes, a seventh-grade shot putter, was not so sure. He took the question to his science teacher, Ms. Ifeoma Chukwu, and they planned a test with steel wool. Steel wool is mostly iron, and its thin strands rust much faster than a solid shot put.

2Ms. Chukwu gave Tobias a simple model of rusting: iron atoms combine with oxygen atoms from the air to form iron oxide. Water speeds up the reaction. The model can be written as 4 Fe + 3 O₂ → 2 Fe₂O₃. In this reaction, every 7.0 g of iron that rusts combines with 3.0 g of oxygen to form 10.0 g of iron oxide.

3Tobias pulled apart six pads of steel wool with the same dry mass, 5.00 g each. He dipped each pad in tap water and let it drip. Five pads went into open dishes on a lab shelf. On each test day, Ms. Chukwu took one of those pads, dried it in a warm oven and let it cool, and Tobias weighed it. The sixth damp pad went into a 1-liter glass jar, which they sealed with a tight lid. On each test day, Tobias weighed the whole jar with everything inside it, without opening it (Table 1).

4On day 14, they opened the jar. Only part of the pad inside had turned orange-brown; the rest of its strands were still gray and shiny. After drying, that pad had a mass of 5.28 g. Ms. Chukwu pointed out that 1 liter of air holds only about 0.28 g of oxygen. The pads in the open dishes, by contrast, were orange-brown almost all the way through.

Table 1. Steel Wool Pads (each 5.00 g dry at the start)
Day	Open-dish pad, dried (g)	Sealed jar, total (g)
0	5.00	412.80
1	5.24	412.80
3	5.62	412.80
7	6.05	412.80
14	6.38	412.80

1.

Use Table 1.

From day 0 to day 14, the dried open-dish pad gained g.

2.

The open-dish pads got heavier as they rusted. Which explanation of the extra mass fits the model in paragraph 2 best?

A. Heat from the drying oven was trapped in the rust and added mass.B. Each iron atom grew larger and heavier as it turned orange-brown.C. Oxygen atoms from the air joined iron atoms to form iron oxide.D. New iron atoms formed from the tap water left in the damp strands.

3.

Use paragraph 4 and Table 1. The sealed jar's total mass stayed the same, but the pad inside it gained mass.

The pad in the jar gained g, so the air sealed inside the jar must have lost g.

4.

By day 14, the pad in the sealed jar had gained much less mass than the open-dish pad. Which explanation fits paragraph 4 best?

A. The jar held about 0.28 g of oxygen, and once it was used, no more could reach the iron.B. The damp air in the jar kept the iron wet, and water slows down the rusting reaction.C. The glass of the jar took in some of the rust's mass, so the scale showed less on the pad.D. Some of the rust in the jar was destroyed, so the pad kept only part of the mass it gained.

5.

Which result is the best evidence that no mass was created or destroyed as the iron rusted?

A. The open-dish pads were orange-brown almost all the way through.B. Part of the pad in the jar was still gray and shiny on day 14.C. The open-dish pad grew from 5.00 g to 6.38 g in 14 days.D. The jar stayed at 412.80 g while the pad inside gained 0.28 g.

Counting atoms and grams

Use the model in paragraph 2: 4 Fe + 3 O₂ → 2 Fe₂O₃.

6.

Count the atoms on each side of the model. Each Fe₂O₃ has 2 iron atoms and 3 oxygen atoms.

Reactants: iron atoms and oxygen atoms. Products: iron atoms and oxygen atoms.

7.

Use paragraph 2 and your answer to question 1. Assume all of the mass the open-dish pad gained was oxygen. Round each answer to the nearest hundredth.

About g of iron had rusted by day 14, forming about g of iron oxide.

8.

Suppose Tobias repeats the sealed-jar test with the same kind of 5.00 g pad, but in a 2-liter jar. Again, the oxygen in the jar runs out before the iron does.

Predict the dried pad's mass on day 14: about g

9.

Suppose the 1-liter jar had stayed sealed for 30 days instead of 14. What would the dried pad's mass most likely be?

A. About 6.38 g, like the open-dish pad, because it had more time to rustB. About 5.28 g, because the oxygen in the jar had already been used upC. Less than 5.00 g, because the rust slowly crumbles into nothingD. About 5.56 g, because the pad keeps gaining 0.28 g every 14 days

Back to the shot puts

Use everything on this sheet to answer.

10.

A shot put has a mass of 4,000.0 g. It rusts in the damp shed for a month, and the rust stays stuck to it. Which prediction fits the steel wool results best?

A. Its mass goes up a little, because oxygen atoms from the air join its iron.B. Its mass goes down a little, because the rust eats away some of its iron atoms.C. Its mass goes up a lot, because each iron atom turns into a heavier iron atom.D. Its mass stays at 4,000.0 g, because a solid ball has no way to take in atoms.

11.

A teammate looks at Table 1 and says, “The sealed jar's mass never changed, so the steel wool inside it did not react at all.” Do you agree? Write a claim, use evidence from Table 1 and paragraph 4, and use the atom model in paragraph 2 to explain your reasoning.