This blog’s focus on matters of energy and climate frequently leads to discussions of thermodynamics. One aspect of that not yet mentioned is latent heat: the energy involved in phase changes of matter. While it takes 1 calorie (not one kilocalorie, as what people call food ‘calories’ are) to heat 1 ml (1 gram, 1 cubic centimetre – don’t you love metric) one degree Celsius, it takes a lot of energy to change that 1 mL of 100˚C water into 101˚C water vapour. Indeed, it takes 540 calories to induce the phase change (turning 1 g of ice into 1 g of water takes 80 calories).
An entertaining way to see this demonstrated is to watch Julius Sumner Miller (mentioned before) talk about temperature. Another is to watch an episode of James Burke’s The Day the Universe Changed: Credit Where It’s Due. As a bonus, it explains how religious dissenters helped to kick off the coal-fired Industrial Revolution in England, eventually generating the climate change problems that confront us so dauntingly now. There is also a fair bit of talk about banking, and the role it played in industrial development.
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Indeed, latent heat is a very useful thing to understand. It is the reason sweat is cooling, that it takes longer for a pot to boil dry than it does to boil, etc.
Does it take as much energy to evaporate one gram of non-boiling water as to convert one gram of hundred degree water to water vapour?
Well, it’s not the same and I can’t find the equation to see if it’s more or less. Basically, there are polynomial equations for latent heat where you can plug in the temperature and it spits out a value. The answers over small temperature ranges never vary greatly, and so my feeling is that the latent heat of vaporization at, say, body temperature is slightly more than at 100ºC.
It can be helpful to view enthalpy (heat due to temperature) as kinetic energy, and latent heat as potential energy that you get back when the water (or whatever) re-condenses.
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