Question
Explain why, ice at $0^\circ C$ is more effective in cooling than water at the same temperature.

Answer

This is due to the fact that for melting, each kilogram of ice takes its latent heat of $3.34 \times 105$ joules from the substance and hence cools the substance more effectively. On the other hand, water at $0^\circ $ cannot take any such latent heat from the substance.

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