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What is the difference between gas and vapour?

  1. Feb 7, 2008 #1
    what is the difference between vapour and gas?

    when gas condence it change in liquid.....so what difference between gas and vapour?
  2. jcsd
  3. Feb 7, 2008 #2


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    The term vapour is used to describe the state of a substance when it's gaseous phase is in equilibrium with it's liquid or solid phases, below it's boiling point.
  4. Feb 7, 2008 #3


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    Feeling the thin ice crunching beneath my feet:
    In a vapour, significant cross-effects will be observable due to the tight mixing of the two phases of the material (i.e, gas and liquid phases).
    Therefore, you must treat it as a multiple phase flow problem, whereas for a "gas", you can treat it as a monophase problem.

    Multiple phase flow calculations is generally beyond ordinary nastiness..
  5. Feb 7, 2008 #4

    Andy Resnick

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    A gas is a single well-defined thermodynamic phase, wheras a vapor is a mixture of two phases (generally gas and liquid). A cloud is made of water vapor, the clear sky contains gaseous water. Steam is vaporized water.

    When a gas condenses to liquid, two physical processes are possible. In one, the phase change occurs via nucleation and growth- small liquid drops spontaneously form and evaporate, but if a drop is larger than some critical radius, it will continue to grow ('nucleates'). This is the most common phenomenon of a gas-liquid (or liquid-solid) phase transition. Less common is 'spinoidal decomposition', which leads to coexistence of a gas and liquid phase- critical opalescence. I don't know if a spinoidal decomposition can occur in the liquid-solid transition.
  6. Jul 22, 2011 #5
    What role does critical temperature play in distinguishing between vapor and gas? I understand that at 374 degrees Celsius and pressure of 218 atmospheres, water reaches its "critical point." Any temperature exceeding the critical temperature of 374 degrees Celsius results in vapor rather than liquid no matter how high the pressure. Oxygen, which is a gas at room temperature, has a critical point at -119 degrees Celsius and 50 atmospheres of pressure. So aside from different critical points, I fail to see much difference between water vapor and oxygen, normally thought of as gas.

    So if I understand you correctly, vapor must coexist with liquid in a mixture of phases. Gas, on the other hand, does not normally coexist with a liquid. Is that correct?

  7. Jul 22, 2011 #6
    The two terms are not always used in a precise manner--even among scientists. Let me approach it from the standpoint of kinetic gas theory:

    All vapors are gases, but not all gases are vapors. Vapors are gases whose mean temperature is below the critical point. In other words, vapor can coexist with their other phases. Water vapor can coexist with liquid water and with ice. In the free atmosphere, it does so all the time.

    Water vapor is found in the atmosphere at all known atmospheric temperatures. Liquid water is found from about -43°C to a little over 100°C in deep mines. Ice is found from 0.01°C on down. All three phases of water can be found in the range of atmospheric temperatures from 0.01°C to somewhere near -43°C.

    This anomalous behavior is what makes water such an interesting topic of study.
  8. Jul 22, 2011 #7
    I thought about your point of vapor coexisting with other phases while gas does not coexist with other phases. I had some trouble coming up with an example of such a gas, though. Oxygen, for instance, can be a liquid at very low temperatures and also a gas can it not? Is there a vaporization curve for oxygen in which liquid oxygen coexists with oxygen as a gas?

    I suppose not.

  9. Jul 23, 2011 #8
    Liquid oxygen will coexist with oxygen vapor right up to the boiling point of liquid oxygen at the pressure at which it is being stored. As a matter of fact, if you have a free surface to the liquid oxygen, you can be sure that oxygen vapor exists above that surface.

    A gas above its critical point does not coexist with its liquid phase because no liquid phase can exist above the critical point.
  10. Jul 23, 2011 #9
    I think that either you are confused or you are using a very, very old textbook. Water vapor is a gas, period. It is invisible. When you see your breath on a cold morning, you are seeing condensate--liquid water droplets. The use of vapor to describe such condensate was discontinued more than a century ago. Along the same lines, the visible "steam" from a kettle is not steam at all. It is condensate. Steam, as water vapor above the boiling point, is invisible.

    Clouds contain water vapor, but the visible portions of clouds are made up of liquid water droplets or solid ice crystals.

    Check Wikipedia or any good encyclopedia.
  11. Jul 23, 2011 #10
    Not necessarily. Water vapor can be found in virtually every portion of the free atmosphere, and it is only occasionally in equilibrium with either its liquid or solid phases. If it were in equilibrium, it would have to be at the equilibrium (saturation) vapor pressure. In most cases, the ambient vapor pressure is much less than the equilibrium vapor pressure; i. e., the relative humidity is less than 100%.
  12. Jul 23, 2011 #11
    Isn't that the same case for water vapor? Its vapor does not coexist with its liquid above its critical temperature? :confused:

    Here's what http://wiki.answers.com/Q/What_is_t...hat_is_the_differance_between_gas_and_vapour" says about the difference between gas and vapor.

    • The word vapor in its natural state is a solid or liquid at room temperature. However, a gas in its natural state at room temperature would still be a gas. Example: 1) steam would be a vapor because at room temperature, it would be water, which is a liquid. 2) Nitrogen (a gas) at room temperature would still be in a gaseous state.
    • To make it more simple, a vapor is a substance which has experienced a phase change. Whereas, a gas is a substance which has not, and will not experience a phase change.
    • Gas is a state of matter while vapor is not.
    • A gas is a substance above its critical temperature but below its critical pressure, while a vapor is a substance above its boiling point temperature.
    • A vapor is a gas. To me, the word, "vapor" suggests a gas that was formed by evaporation of something that is a liquid at room temperature. For example, water vapor. It also connotes a gas that can be seen.

    I think I'll get it eventually.

    Last edited by a moderator: Apr 26, 2017
  13. Jul 23, 2011 #12
    Good Catch! Pedantically speaking, the proper term for water vapor at temperatures above the critical temperature would be "water gas". However, "water vapor" has a long history of scientific use for gaseous water at all temperatures.

    I looked at your source, and quite frankly JEK does not know what he (or she) is talking about. If you like Wikis, try: http://en.wikipedia.org/wiki/Water_vapor

    Specifically, JEK says:

    "a vapor is a substance which has experienced a phase change. Whereas, a gas is a substance which has not, and will not experience a phase change." Absolute tommyrot! All gases will change phase if the temperature is low enough.

    "Gas is a state of matter while vapor is not." The accepted scientific usage is "phase of matter". In physics, a state and a phase are two different things.

    "A gas is a substance above its critical temperature but below its critical pressure, while a vapor is a substance above its boiling point temperature." Both statements are false.

    "It also connotes a gas that can be seen." Maybe it does to JEK, but it does not do so in physics.
  14. Sep 6, 2011 #13
    So, is steam a 'vapour of water' or a 'gas of water'?
  15. Sep 6, 2011 #14
    That depends upon how you define "steam". If you use the engineering definition of water vapor above the critical point, it is both. If you refer to the visible emanations from a tea kettle, it is neither. What you see at the mouth of a tea kettle is liquid water droplets. Water is invisible in its gaseous phase, whether you call it vapor or gas.
  16. Sep 6, 2011 #15


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    Wet steam or dry steam?
  17. Sep 7, 2011 #16
    Let me rephrase....correct me if i am wrong..

    1. Wet Steam is a 'vapour of water'.
    2. Dry saturated steam (also superheated steam) is a 'gas of water'.
  18. Sep 7, 2011 #17


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    I am ready to agree, but I don't think these terms are used precisely, so in reality whenever in doubt, you should clarify what you mean (or what someone meant).
  19. Sep 7, 2011 #18
    I don't know what you mean by either term. I accept the engineering definition of steam to mean water vapor or gaseous water at temperatures above the critical point. You can call it a gas or call it a vapor. You will be correct in either case. There is no meaningful distinction between the two in scientific usage.

    If by "wet" steam you mean what you see coming out of the spout of a tea kettle, then that effusion contains water in two phases: the gaseous phase (invisible) and the liquid phase (visible).

    Let me iterate. Water vapor is water gas always. The two are steam when the temperate is above the critical point.

    I think many posters are trying to use common everyday speech definitions as scientific definitions. This never works and simply leads to endless haggling. The whole purpose behind the use of scientific terminology in scientific discussions is to avoid this fruitless activity.

    Most of these posts would never have been written if the writer had taken the trouble to look up the terms in a reputable scientific reference source. Here's a start:

    Note that wiki considers (as I do) that deposition is simply one form of condensation; just as sublimation is simply one form of vaporization.
  20. Sep 7, 2011 #19


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    How so? Vapor is defined as a substance in the gas phase at a temperature lower than its critical point. I can easily see where the confusion comes from.
  21. Sep 7, 2011 #20
    I agree with you, Drakkith, that the source of confusion is readily apparent in everyday speech. However, I don't see why there should be any confusion at all on a scientific forum. The terms gaseous water, water vapor, and steam all have precise and accepted scientific meanings. Once those meaning have been posted on a forum that should be the end of it.
  22. Sep 7, 2011 #21


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    Both terms are extensively used by engineers:


    Dry steam search doesn't give as obvious results, but there are enough hits to prove it is not a term I made yesterday.

    It would be nice if all trades were using the same terms and apply the same meaning to them, unfortunately that's not the case. For astrophysicists oxygen is a metal.
  23. Sep 7, 2011 #22
    In engineering terms the distinction between wet and dry steam is crucial.

    This is because the amount of liquid water in the steam, as measured by the dryness fraction, affects the energy budget in steam machinery. Engineers, after all, are primarily interested in steam as a 'prime mover'.

    In particular

    specific enthalpy of wet vapour = h = hf + xhfg in steam tables,

    where x is the dryness fraction.
  24. Sep 7, 2011 #23
    As to the original question:

    A vapour is a gas that is below its critical temperature and can therefore be liquified by a suitable increase in pressure.

    It is not necessary for there to be any liquid present.
  25. Sep 7, 2011 #24
    The terms gas and vapor refer to the same physical phase. However, "vapor" is the term used when the substance is at a temperature less than its critical temperature, and "gas" is the proper term to use when the substance is at or above its critical temperature. Note that a "vapor" cannot exist at pressures at or above the critical pressure, but a "gas" can. Finally, increasing the pressure of a vapor at constant temperature can cause the vapor to condense to liquid; but a gas will not condense if its pressure is increased in such a fashion, and even though it may be compressed to become a dense phase fluid (supercritical), even one with a liquid-like density, we would still consider it to be a gas.
  26. Sep 8, 2011 #25
    An engineer working with a steam engine would be very unhappy to find himself working with anything less than pure, gaseous water. That's why steam engines have things like steam strainers to make certain no liquid gets into the steam main, which is called the "dry pipe" for this reason. That's also one reason why steam engines contain superheater tubes, whose purpose is to raise the pressure of the steam well above the boiling point of water (recapturing what would otherwise have been waste heat from the firebox in the process) in addition to increasing its pressure, and, thus, the mechanical potential energy it contains which can then be released as mechanical kinetic energy against the piston head or turbine blades, depending on the type of engine you're powering.

    "Toot! Toot!"

    Last edited: Sep 8, 2011
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