Final answer:
Ice at 0°C is more effective at cooling than water at the same temperature. It absorbs more energy due to the latent heat of fusion required for the phase change from solid to liquid, which occurs without a temperature increase.
Step-by-step explanation:
A bag containing 0°C ice is more effective in absorbing energy than one containing the same amount of 0°C water due to the latent heat of fusion. The process of melting ice requires a significant amount of heat to change its state from solid to liquid without changing its temperature. This is known as heat transfer and is crucial when considering the cooling effect of ice versus water.
When ice at 0°C melts, it absorbs a great amount of heat from the surroundings. This is because the heat is used to break the molecular bonds that keep water in its solid state, rather than to increase the temperature of the substance. As a result, ice can absorb more heat before it warms up compared to liquid water at the same temperature, which makes ice a more effective coolant.
On the other hand, once the ice has completely melted, the water's temperature can start to rise. As long as there is a mixture of ice and water, the temperature will stay at 0°C, maintaining thermal equilibrium. Only when all the ice is melted will the temperature of the water start to increase.
To compare, evaporating liquid nitrogen would absorb a significantly larger amount of heat due to its lower boiling point and the high latent heat of vaporization, but for practical cooling purposes at 0°C, ice is a very effective substance for dissipating the heat.