what is the difference between gas and vapour?

pawan_ctn

what is the difference between vapour and gas?

when gas condence it change in liquid.....so what difference between gas and vapour?

Hootenanny

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.

arildno

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..

Andy Resnick

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.

Jagella

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?

Jagella

klimatos

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.

Jagella

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.

Jagella

klimatos

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.

klimatos

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.

klimatos

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%.

Jagella

Isn't that the same case for water vapor? Its vapor does not coexist with its liquid above its critical temperature?

Here's what one source 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.

Jagella

klimatos

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.

renjith_p

So, is steam a 'vapour of water' or a 'gas of water'?

klimatos

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.

Borek

Wet steam or dry steam?

renjith_p

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'.

Borek

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).