A chunk of ice whose temperature is –20C is added to an insulated cup filled with water at 0C. What happens in the cup?

Question

BlackTom AI · Accepted Answer

When an ice chunk at –20°C is placed into 0°C water inside an insulated cup, energy transfer occurs between the water and the cold ice.
Option 1: 'The ice melts until it reaches the temperature of the water.' This would require the ice to absorb enough heat to melt, but the ice is at –20°C and must first warm to 0°C before melting can even begin. The water, at 0°C, would need to supply that latent heat of fusion to melt ice, which is not simply guaranteed in this setup; more importantly, the presence of ice at a lower temperature tends to cause some water to freeze rather than all ice melting immediately. Therefore this option misstates the process by skipping the necessary heating of ice to 0°C and assuming only melting.
Option 2: 'The water cools until it reaches the temperature of the ice.' In an insulated system, the final temperature will be 0°C, not below, because the liquid water at 0°C cannot cool below its freezing/melting point without freezing, and any further cooling would require the formation of more ice at 0°C. This option contradicts the typical 0°C equilibrium scenario where both phases settle at 0°C with some ice present.
Option 3: 'Some of the water freezes, so the chunk of ice gets larger.' As heat flows from the water to the colder ice, the water can lose enough energy to start freezing some of its molecules, increasing the amount of ice and making the chunk larger. This aligns with energy conservation and the tendency for the system to move toward a state with more solid ice at 0°C while the remaining water remains at or near 0°C.
Option 4: 'Some of the ice melts and the water cools at the same time.' While some melting could occur under certain conditions, in this setup the dominant process is water freezing onto the ice rather than vice versa, because the ice starts far below 0°C and can absorb heat to raise to 0°C and then to melt only if sufficient energy is available from the water. This option generalizes a simultaneous melting and cooling that doesn’t accurately reflect the typical outcome where freezing of water on contact with the cold ice increases the ice mass.

A chunk of ice whose temperature is –20C is added to an insulated cup filled with water at 0C. What happens in the cup?

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