The amount of heat required to change a liquid to gaseous state without any change in temperature is known as
- A.specific heat capacity
- B.mechanical equivalent of heat
- C.latent heat of vaporization
- D.quenching
Correct answer
Explanation
The correct answer is C, latent heat of vaporization. It is the heat a liquid absorbs to turn into vapour at its boiling point, while its temperature stays the same.
During boiling, the heat supplied does not raise the temperature; it is used to break the forces holding the molecules together in the liquid. For water, the latent heat of vaporization is about 2,260 kJ per kg (540 cal per g), much larger than the latent heat of fusion of ice, about 336 kJ per kg. This is why a burn from steam at 100 degrees C is worse than one from boiling water: the steam releases this extra heat when it condenses on the skin. Sweating cools the body for the same reason.
A is wrong because specific heat capacity is the heat needed to raise the temperature of 1 kg of a substance by 1 degree, so it involves a temperature change. B is wrong because the mechanical equivalent of heat, about 4.186 joules per calorie, links work and heat. D is wrong because quenching is the rapid cooling of hot metal in water or oil to harden it.
Exam tip: latent heat changes state at constant temperature; specific heat changes temperature.
Practice Questions
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