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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First, consider the physical setup: a chunk of ice at -20°C is placed into an insulated cup containing water at 0°C. Heat will flow from the water (warmer) to the ice (colder) until thermal equilibrium is reached, with the constraint that water can only be at or above 0°C unless it freezes, and ice can heat up to 0°C before any melting occurs.

Option 1: The ice melts until it reaches the temperature of the water.
This statement suggests the ice will melt simply because it is in contact with 0°C water. However, the ice is initially at -20°C and must absorb heat to warm to 0°C before any melting can occur. The early stage is heating of the ice, not melting. Only after the ice reaches 0°C could melting begin. Therefore, this option is incomplete or misleading as a description of the full process, since it implies melting from the start rather than initial warming of the ice.

Option 2: The water cools until it reaches the temperature of the ice.
Water at 0°C cannot sustainably cool below 0°C while remaining liquid; doing so would require it to freeze. Since the presence of ice enables freezing, the system would instead drive some water to freeze to supply latent heat to raise the ice to 0°C, not to simply cool the liquid water to match the ice’s temperature. This makes the statement incorrect as a complete description of what happens.

Option 3: Some of the water freezes, so the chunk of ice gets larger.
This is the physically correct mechanism. Heat flows from the 0°C water to warm the ice from -20°C up to 0°C. To provide enough energy to bring the ice to 0°C (and to satisfy energy balance in an insulated system), some of the liquid water must freeze at 0°C, which releases latent heat of fusion. That latent heat helps raise the ice temperature to 0°C and increases the total amount of ice. Thus, the chunk of ice grows as some water freezes.

Option 4: Some of the ice melts and the water cools at the same time.
At first glance this seems plausible in a 0°C interface, but melting of ice would absorb heat from the water, counteracting the cooling of the water. Since the ice starts at -20°C, its initial response is to warm up to 0°C, not to melt. The dominant energy transfer in this scenario is the freezing of some water (releasing latent heat) rather than melting of ice, so this option does not accurately describe the primary process.

In summary, the key idea is that the cold ice draws heat from the water, the ice warms to 0°C, and, to conserve energy in an isolated system, some of the water must freeze, causing the ice chunk to grow. Therefore, the correct description is that some of the water freezes, so the chunk of ice gets larger.

Chemistry (H) - Varnold - Preparatory Academy - 2nd Semester - 2024/2025 2025 Thermochemistry Practice Test

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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