4

I've learned in school that it takes 2260 kJ/kg for water to go from boiling to steam (for the phase shift)

However, when I mop the floor during winter it becomes dry rather quickly because the humidity is so low. And it evaporates although the water/floor is cold (~15°C to 18°C, I'd guess).

How is the water in the air different from steam? (I guess one point is that the air can be "saturated" at some point, but you can always add more steam ... but I still don't quite get the difference as both should be isolated water molecules)

Is the energy required to go from water to steam different depending on the humidity?

(I'm sorry, I don't even know how to phrase the title properly. I guess the part that confuses me is where those 2260 kJ/kg come from in dry+cold air ... or if that 2260 kJ/kg does not always apply)

Martin Thoma
  • 997

1 Answers1

5

In brief, the ~2260 kJ/kg (somewhat dependent on the temperature) latent heat of vaporization is always available to evaporate water—when it's thermodynamically favorable. Surroundings of even a relatively cool (to us) temperature of ~15°C to 18°C contain plenty of available thermal energy.

When is it—evaporation or sublimation—thermodynamically favorable? When the chemical potential or activity is lower in the gas state than in the condensed state. Pure liquid or solid water has an activity of 1, whereas gaseous water's activity corresponds to the relative humidity. As long as the relative humidity is less than 100%, water evaporates, and the surroundings must provide the associated latent heat. We can trace the reason back to the Second Law: The entropy increase from individual molecules detaching from a condensed phase and exploring the surroundings more than pays for the environmental cooling that's incurred, until the relative humidity reaches 100%. In kinetic terms, more water molecules evaporate than condense until the relative humidity reaches 100%, and the detaching molecules carry higher-than-average individual energies.

I guess one point is that the air can be "saturated" at some point, but you can always add more steam

Here one must distinguish between "steam" as gaseous water and "steam" as the colloquial term meaning visible condensed liquid water droplets. If you heat water under higher humidity, the vapor more readily condenses directly above. (The air above also tends to heat up as mediated by heat transfer with the broader surroundings.)

Chemomechanics
  • 30,163