Melting Ice in Two Cups
Two identical cups, X and Y, each hold 50 g of crushed ice at 0 °C. Cup X is wrapped in a dry towel; cup Y is left bare on the table. After 10 minutes, X still contains some ice, while Y contains only water. The room temperature is 28 °C throughout. A pupil claims that the towel 'makes cold' and that is why X keeps its ice longer.
1. Explain, in terms of heat, why cup X kept its ice longer than cup Y.
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Ice melts when it gains heat from the surroundings. The towel is a poor conductor of heat, so it slows the transfer of heat from the warm room air to the ice in X. Cup Y has no such barrier, so heat reaches its ice faster and all of it melts within 10 minutes.
The towel does not create cold. It only slows heat gain. The ice itself stays at 0 °C while melting, so the towel is not making the ice colder; it is reducing how quickly heat arrives.
2. Predict what would happen if the towel around X were soaked in water before wrapping. Give a reason.
Read the worked answer
The ice in X would probably melt faster than with a dry towel. A wet towel conducts heat better than a dry, fluffy one because water fills the air spaces that normally slow heat transfer. Water also evaporates, but evaporation takes heat from the towel and its surroundings, and the wet towel is in close contact with the cup, so the overall effect is likely faster melting.
A fair comparison would use the same starting mass of ice, the same room temperature, and towels of the same size, changing only whether the towel is wet.