Final answer:
The heat required to melt 1 kg of ice at 0°C is 334 kJ, and the heat released by 1 kg of water when it freezes at 0°C is also 334 kJ.
Step-by-step explanation:
The heat required to melt 1 kg (or 1.000 kg) of a substance, specifically ice at 0°C, is based on its latent heat of fusion. The latent heat of fusion for water is 334 kJ/kg, which means that the heat required would be Q = (1.00 kg)(334 kJ/kg) = 334 kJ. This is the amount of energy needed to disrupt the intermolecular forces in the ice without changing its temperature.
Conversely, the heat given off when 1 kg of the same substance freezes, which for water is also at 0°C, is exactly the same as the heat absorbed when it melts, due to the fact that phase changes are reversible processes. Therefore, 1 kg of water at 0°C would release 334 kJ of energy when it freezes to become ice at 0°C.