Latent heat is the amount of heat absorbed or evolved by 1 mole, or a unit mass, of a substance during a change of state (such as fusion, sublimation or vaporisation) at constant temperature and pressure.
The Scottish physicist and chemist Joseph Black discovered latent heat in 1761 after reading an observation by the German scientist Daniel Gabriel Fahrenheit that “water would sometimes grow considerably colder than melting snow, without freezing, and would freeze in a moment, if disturbed, and in the act of freezing emitted many degrees of heat.” He experimented upon this in the following manner:
- He used two thin globular glasses: in one he froze 142 g of water into ice and in the other he poured the same amount of water at a temperature of 0.56°C.
- He suspended them in mid-air and kept a distance of 45.7 cm between them.
- He measured the temperature of the air around the glasses to be 8.33°C.
- He placed a thermometer in the glass of water and in less than half a minute, its temperature reached the temperature of the water. Afterwards, its temperature rose every 5 or 10 minutes for half an hour until the water reached a temperature of 4.44°C.
- The ice in the other glass, a few minutes after the start of the experiment, started to melt and that is when he noted the time and left the glass for 10.5 hours after which he found a small amount of ice remaining. Once the remaining ice melted after a few minutes, he measured the temperature of the water near the glass to be 4.44°C. This is of importance because the water near the sides of the glass was warmer than the water that had just formed from the melted ice meaning that heat from the melting ice was transferred more to the water near the sides of the glass rather than the other way around.
From this experiment, it was learned that the ice in the glass received the same amount of heat in 21 half-hours that the water in the other glass received in a single half-hour since the temperatures of both the ice and the water were the same at the end of the experiment. By all these observations, he calculated that the amount of heat that the ice received should have increased its temperature by 81.5°C ((4.44°C – 0.56°C) x 21 half-hours = 81.5°C). Since this huge temperature increase was not observed, Joseph Black proceeded to call this “latent” (Latin for “hidden”) heat.
Here is an engraving of Joseph Black:
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