Cocoa on the Ice: Following the Atoms in a Camp Stove

Use mass data and an atom model to show that mass is conserved when fuel burns.

The camp stove

Read the passage and study Tables 1 and 2. Then answer the questions.

Hot Cocoa on a Frozen Lake

1Every February, seventh grader Wren Sokolowski goes ice fishing with her uncle, Mr. Anselm Sokolowski, on a frozen lake near their town. On a −10 °C morning they sat beside two holes drilled through 30 cm of ice. To stay warm, they made hot cocoa on a small camp stove. Her uncle set up the stove outdoors on a flat board, away from the fishing shelter, because burning fuel uses up oxygen and gives off gases that should not build up in a closed space.

2The stove burns propane, which is stored as a liquid under pressure in a small steel cylinder. Wren's science class had been studying chemical reactions, so she kept notes. When propane burns, it reacts with oxygen gas from the air. Her teacher showed the class a model of the reaction, C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O, and gave the masses that go with it (Table 2).

3Wren weighed the propane cylinder on a kitchen scale before and after the trip. Her uncle packed the pot with clean snow, set it on the stove and lit the burner. Right away, drops of water formed on the outside bottom of the cold pot and dripped onto the board, even though the outside of the pot had been dry. The drops stopped forming once the pot warmed up. The snow melted, and they heated the water for two mugs of cocoa.

4In the afternoon, they built a small campfire in a metal fire ring on the shore. Wren weighed the dry firewood before it went into the ring. When the fire had burned out and cooled, she scooped out the gray ash and weighed it. No unburned pieces of wood were left in the ring. Her cousin Milo looked at the small pile and said, "The fire destroyed almost all of the wood."

Table 1. Wren's Measurements
Item	Before (g)	After (g)
Propane cylinder	860	838
Firewood → ash	6000	60

Table 2. Masses in the Propane Reaction (for every 44 g of propane that burns)
Substance	Reactant or product	Mass (g)
Propane (C₃H₈)	Reactant	44
Oxygen (O₂)	Reactant	160
Carbon dioxide (CO₂)	Product	132
Water (H₂O)	Product	72

Use Table 1. Assume all of the mass the cylinder lost was propane that burned.

The stove burned g of propane during the trip.

Use Table 2 and your answer to question 1. The masses in a reaction change in the same ratio as the amount of propane that burns.

Burning that propane used g of oxygen and formed g of carbon dioxide and g of water.

Use your answers to questions 1 and 2.

Total mass of the reactants: g. Total mass of the products: g.

The cylinder lost only 22 g, yet the stove gave off more grams of products than that. Where did the extra mass come from?

A. From oxygen in the air, whose atoms joined the propane's atoms
B. From the snow in the pot, which mixed into the gases from the burner
C. From the flame's heat, which added mass to the gases as they formed
D. From new atoms that the reaction made as the propane burned away

Use paragraph 3. Which explanation of the drops on the outside of the cold pot fits the reaction model best?

A. The flame changed some of the steel on the bottom of the pot into water.
B. Water from the melting snow soaked out through the bottom of the steel pot.
C. Some propane leaked out of the burner and turned into drops of water.
D. Water formed in the flame and turned back into a liquid on the cold metal.

Counting the atoms

Use the model C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O. Each CO₂ has 1 carbon atom and 2 oxygen atoms; each H₂O has 2 hydrogen atoms and 1 oxygen atom.

Count the atoms in all of the products: 3 CO₂ and 4 H₂O.

The products hold carbon atoms, hydrogen atoms and oxygen atoms.

Where did the oxygen atoms in the water drops on the pot come from?

A. From the propane molecules, which each hold oxygen atoms
B. From oxygen molecules in the air, which were taken apart
C. From the steel cylinder, which gives off oxygen as it warms
D. From the carbon dioxide, whose oxygen atoms were made in the flame

Wren builds the reaction with beads. She starts with 1 propane model (3 carbon beads and 8 hydrogen beads) and 5 O₂ models (2 oxygen beads each). She builds 3 CO₂ models and only 3 H₂O models.

Wren has hydrogen beads and oxygen bead left over, which is enough for one more H₂O model.

Underline each statement that the propane model and Table 2 support.

Each carbon atom from the propane ends up in carbon dioxide.Burning propane makes new hydrogen atoms in the flame.Oxygen atoms from the air end up in both of the products.The products have more mass than the reactants that formed them.The carbon atoms from the propane end up in the water.

The campfire

Use paragraph 4 and Table 1. Wood burns in a reaction with oxygen from the air, and it forms carbon dioxide, water vapor and ash.

10.

The 6,000 g of firewood left 60 g of ash. What was the total mass of the gases and smoke that the fire gave off?

A. About 5,940 g, the mass of wood that did not end up as ash
B. About 60 g, the same as the ash, because a reaction splits mass evenly
C. More than 5,940 g, because oxygen from the air joined the wood's atoms
D. Less than 5,940 g, because heat and light carried some mass away

11.

Milo says, "The fire destroyed almost all of the wood." Do you agree? Write a claim, use evidence from Table 1 and from the stove's reaction, and explain your reasoning with atoms.