When the temperature is same, it means that the kinetic energy of the particles of both phases are the same, so what is the difference between a vapor and liquid of a substance, such as ethanol, when they are in the same temperature?

  • $\begingroup$ The vapor is in gas phase, so it is a gas. $\endgroup$ – Peter Diehr May 18 '18 at 9:18
  • $\begingroup$ well, they are at different density to begin with. As a result, their local structure is different, which also deeply affects the kinetics. $\endgroup$ – lr1985 May 18 '18 at 10:48
  • $\begingroup$ I think you mean "liquid", not "fluid". A gas is also a fluid. $\endgroup$ – Bzazz May 18 '18 at 11:28
  • $\begingroup$ A vapor is a suspension of liquid particles in a gas. Ethanol in air can exist as a vapor and as a mixture, depending on how the ethanol is introduced into the air (such as atomization) and what the temperatures are of the ethanol and air. It cannot exist for long as a vapor if the air is not fully saturated with gaseous ethanol. $\endgroup$ – docscience May 18 '18 at 11:47
  • $\begingroup$ Their specific heat capacity is different. :) $\endgroup$ – Bettertomo May 18 '18 at 12:55

As silly as it sounds, the difference is that one is a gas, and the other is a liquid. The vapour has the properties of a gas, i.e. it expands as much as it likes, it compresses according to certain laws, etc. The liquid has different properties. Effectively, they will look different, as you imagine a liquid and a gas being different. The gas will be a gas, the liquid a liquid.

The problem here, is that you think the phase of a substance is a function of its temperature only. This is not necessarily the case. I can heat ice up to 0 C, and it will still be ice. Conversely, I can cool water down to 0 C, and it will still be water. To have it actually go across the critical point, I have to spend some additional energy, called latent heat.

A critical point is a value of temperature (and pressure, and whatever else) where the two states of matter coexist.

  • $\begingroup$ What kind of energy is latent heat? $\endgroup$ – ilia May 18 '18 at 13:06
  • $\begingroup$ For example, it may be needed to break the bonds of the crystalline lattice, in the case of a solid to liquid transition. $\endgroup$ – Bzazz May 19 '18 at 1:38

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