The Melting Ice Debate
Two students each put 50 g of ice at 0 °C into identical cups. Student 1 leaves the cup on the lab bench at 22 °C. Student 2 wraps an identical cup in a thick wool sock and leaves it on the same bench. After 15 minutes, Student 1's ice has almost completely melted; Student 2's ice has melted only slightly. Student 2 says, "The wool sock must be adding cold to the ice, which is why it melts slower." The teacher asks the class to explain what the wool actually does, using ideas about energy transfer, and to predict what would happen if the wool-wrapped cup started with warm water at 40 °C instead of ice.
1. Explain what the wool sock is actually doing, and correct Student 2's claim that it 'adds cold.'
Read the worked answer
The wool does not add cold-cold is not a substance that flows. The wool is an insulator: it traps air, and air is a poor conductor of heat. Heat energy moves from warmer to cooler regions (bench/room air at 22 °C toward the 0 °C ice). The wool slows that energy transfer, so the ice receives heat more slowly and melts more slowly. Nothing is being 'added'; instead, the rate of energy transfer is reduced. Evidence: both cups sit in the same 22 °C room, so the surroundings are identical-only the insulation differs. The unwrapped cup exchanges heat freely with the room; the wrapped cup is buffered. The correct framing is 'the wool slows heat flow into the ice,' not 'the wool makes the ice colder.'
2. Predict what would happen if the wool-wrapped cup held 40 °C water instead of ice, and explain whether the wool would 'keep it warm' or 'keep it cold.'
Read the worked answer
The wool-wrapped cup of 40 °C water would cool more slowly than an unwrapped cup of the same water. The wool slows heat transfer in either direction-it doesn't have a preferred direction. With warm water, heat flows from the water out toward the cooler room, and the wool slows that loss, so the water stays warmer longer. With ice, heat flows from the room into the ice, and the wool slows that gain, so the ice stays frozen longer. In both cases the wool reduces the rate of energy transfer, keeping the contents closer to their starting temperature. So 'keeps it warm' and 'keeps it cold' are both correct descriptions of the same insulating effect-it resists change, not a particular temperature. This shows the wool is not a source of cold or heat.