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."