Final answer:
The heat required to vaporize 1 kg of a substance is called the heat of vaporization, and the heat given off by 1 kg of the same substance when condensing is also equal to the heat of vaporization but with opposite sign.
Step-by-step explanation:
The heat required to vaporize 1 kg of a substance is called the heat of vaporization, and the heat given off by 1 kg of the same substance when condensing is also equal to the heat of vaporization but with opposite sign. The heat of vaporization is the amount of heat energy required to convert a substance from a liquid to a gas at a constant temperature and pressure.
For example, in the case of water, when 1 mol of water at 100°C and 1 atm pressure is converted to 1 mol of water vapor at 100°C, 40.7 kJ of heat is absorbed from the surroundings. On the other hand, when 1 mol of water vapor at 100°C condenses to liquid water at 100°C, 40.7 kJ of heat is released into the surroundings.