The Melting Ice Race
Four identical ice cubes, each 30 g and taken from the same freezer at −5 °C, are placed in four identical bowls at room temperature (21 °C). Bowl A holds the ice on a dry saucer. Bowl B holds the ice in 100 mL of water at 21 °C. Bowl C holds the ice wrapped in a wool sock. Bowl D holds the ice on a metal tray. Students time how long each cube takes to melt completely. Bowl B melts fastest, then Bowl D, then Bowl A, then Bowl C. A student says, "The wool sock must make heat, because wool is warm." Another says, "The wool slows heat getting to the ice."
1. Which explanation fits the results? Explain using heat transfer.
Read the worked answer
The wool slows heat transfer to the ice. Wool traps air, and trapped air is a poor conductor of heat, so heat from the room reaches the ice slowly. That is why Bowl C melted last. Wool does not make heat; it only slows heat loss or gain. Bowl B melted fastest because water is a better conductor than air and the 100 mL of water was in direct contact with the ice, so heat moved into the ice quickly from all sides. Bowl D melted before A because metal conducts heat faster than the dry saucer. The order B, D, A, C matches how quickly each setup delivers heat to the ice, not how much heat each setup 'makes'.
2. Predict what would happen if the wool sock in Bowl C were wet rather than dry. Explain.
Read the worked answer
The ice in a wet sock would likely melt faster than in a dry sock. Water conducts heat better than trapped air, so a wet sock would carry heat from the room to the ice more quickly. The dry sock works as insulation because of the air trapped in its fibres; once water fills those spaces, the insulating air is replaced by water, and the sock conducts heat better. So the melting time for C would shorten, though it might still be slower than Bowl B, where the ice sits in a full bowl of water.