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Think about it. If there's water in a gaseous form, how can there also be water in a liquid form? The gaseous water would have to have more energy than the liquid, but if it's the same temperature, how would it get that energy?

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    $\begingroup$ What do you know of phase transitions and latent heat? $\endgroup$ – Kyle Kanos Oct 7 '14 at 0:38
  • $\begingroup$ There is a thermodynamic equilibrium between liquid water and water vapor that depends on temperature. $\endgroup$ – CuriousOne Oct 7 '14 at 0:39
  • $\begingroup$ @CuriousOne If it has thermodynamic equilibrium, than how are the phases different? I thought phases of matter depend on the energy level. $\endgroup$ – Joe Pigott Oct 7 '14 at 0:41
  • $\begingroup$ The phases can, for instance, differ in the average distance between the water molecules. The thermodynamic equilibrium is given by maximizing the TOTAL entropy of the system (for a given amount of total internal energy, total volume and total particle count). This means that we have to maximize S_liquid + S_gas, which determines the total number of molecules that will be in the liquid and the gas phase each. At lower temperatures the maximum will have most of the molecules in the liquid phase, at high temperature there will be more and more in the gas phase and fewer in the liquid. $\endgroup$ – CuriousOne Oct 7 '14 at 0:49
  • $\begingroup$ If liquid and gaseous water cannot coexist, there there would never be rain and everything would be extremely dry. $\endgroup$ – LDC3 Oct 7 '14 at 4:31
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The branch of physics that studies these problems is called "thermodynamics", and it is a very successful theory as it describes most bulk matter behavior and can be used in engineering and other projects reliably.

In thermodynamics matter is composed of molecules modeled as classical particles with some collective properties that are defined by measurements.

Matter has been observed to appear in phases:

phases of matter

This diagram shows the nomenclature for the different phase transitions.

The variable on the right is an energy of the system variable, enthalpy.When this energy variable is large enough only gas and plasma (atoms separate into ions and electrons) phases can exist, and in even higher values only plasma. This diagram describes the observations accurately.

The gaseous water would have to have more energy than the liquid, but if it's the same temperature, how would it get that energy?

The connection of temperature in statistical mechanics, at the level of the atoms and molecules that compose matter, is

temperature from average kinetic energy

It is seen that the average kinetic energy of the gas is proportional to the temperature, not to the total kinetic energy of the gas ( the integral of the plot).

gas kinetic energy

Matter at a given temperature will have an energy distribution as some atoms will be fast , some slow but the average kinetic energy will define the temperature. Thus the average kinetic energy of the gas molecules is the same as the average kinetic energy of the atoms in the liquid water . The molecules of water in gas form enter the air because they are at the tail of the kinetic energy distribution of the liquid water, and at the surface can escape the surface tension potential that is defining the surface of the liquid. These as seen can be very few, thus their ensemble has small total kinetic energy, and they reach equilibrium by scatterings with each other in the gas and with surface molecules (the two temperatures, liquid/gas, become equal).

kinetic energy distribution

Each individual molecule takes a bit of kinetic energy leaving and some other molecule returns it falling back on the surface at the equilibrium of the temperature, and the number of gaseous molecules remains stable for a given temperature. Increasing the temperature increases the number of molecules in the tails of the kinetic energy distribution and more water molecules can escape the liquid surface etc.

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    $\begingroup$ A big +1 but one question: what do you mean with the statement "It is seen that the average kinetic energy of the gas is proportional to the temperature, not to the total energy of the gas", specifically what do you mean by "the total energy of the gas"? $\endgroup$ – Floris Oct 9 '14 at 20:09
  • $\begingroup$ @Floris in the context, "the gas should have more energy" seems as if he/she assumes that temperature and energy are proportional, as if the sum of kinetic energies gave the temperature. I will edit. $\endgroup$ – anna v Oct 10 '14 at 3:52
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For a given temperature and pressure, it is possible for a certain amount of liquid to be in the vapor phase. At any moment, water molecules from the liquid escape to the vapor; and at the same time some of the vapor molecules go back into the liquid.

Equilibrium is established when the rate of these two processes is the same. At what (partial) pressure that will happen is a function of temperature.

Example: start at a low temperature. Very few water molecules from the liquid have the energy to escape; so if even just a few molecules per second hit the water and "stay", you have equilibrium at low vapor pressure.

Now increase the temperature of the water. More molecules can escape - initially exceeding the rate at which molecules return to the liquid phase. The partial vapor pressure builds, until the two rates are once again the same.

Once the rate at which the molecules escape the water exceeds the rate at which they return even at full atmospheric pressure, the water "boils". The water molecules returning don't keep up with the rate of them leaving.

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