Difference gas vs vapour 4

[nlkw]

Hi, I've been asked by management to prepare a 'dummies' style note on the fundamental difference between Gas and Vapour. Our company manufactures flow-metering instruments, so I have to make my points relevant.

My boss (not an engineer) has specifically requested not to get too technical, as the info will compiled into a training manual for users who may not be technically-trained. On the other hand, I don't want to underplay the significance between the two.
What would you suggest to be the salient points, in less than 1000 words?

 

[25362]

Usual definitions that may help, in less than 100 words:

Gas - a phase of matter in which the substance expands readily to fill any containing vessel, and characterized by relatively low density.

Vapor - a gas at a temperature below the critical temperature, so that it can be liquefied by compression, without lowering the temperature.

 

[Latexman]

A picture is worth a 1000 words, use a phase diagram and simply label it.

Good luck,
Latexman

 

[zdas04]

Don't know if that would work since there is no distinct lines on a phase diagram to differentiate gas from vapor because they are the same thing.

Maybe a Molier Diagram. I had a prof once who said that a gas is a superheated vapor, and a vapor is a gas at saturation conditions (i.e., in contact with a subcooled liquid). The superheated/saturated distinction is clear on a Molier Diagram. I've never seen anyone else who bought into that distinction 100%.

David

 

[Latexman]

On a phase diagram (P vs. T) it'd be easy to draw in a squiqqly vertical line to the critical temperature and label the diagram to indicate vapor below the critical temperature and gas above the critical temperature. Why squiggly? To indicate somewhat of a gray area.

A Mollier Diagram would visually be good, but does having an axis with enthalpy as a variable fit into the "dummied down" criteria? I'd rather stick with less abstract variables with non-technical folks, but maybe the OP can handle it.

Good luck,
Latexman

 

[mbeychok]

.
I know that others won't agree. Nonetheless, I will state unequivocally that the words gas and vapor are synonyms. They both name the phase of matter which is not solid and not liquid.

Read this article in the online Citizendium encyclopedia:

Click here ==> Gas



Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[zdas04]

I agree completely. A gas is a substance the conforms to the shape of its container and does not exhibit a coherent surface (not sure how you differentiate a gas from a plasma, but that isn't the question). So is a vapor. One interesting paragraph in your referenced article

One way in which the word vapor sometimes replaces the word gas is when the gaseous phase is in equilibrium with the corresponding liquid or solid. We call the pressure of the gas phase in equilibrium with the corresponding liquid phase vapor pressure. We connote the equilibrium of the gas and liquid phases of a substance as vapor-liquid equilibrium. But note that all of the connotations are defined as the equilibrium between a gas phase and a liquid phase.

This seems to support my prof's definition from the 70's.


It is kind of like when I talk about "gas" to my family, they hear "gasoline" regardless of how many times I explain it.

 

[Latexman]

Interesting! At synonyms.com and synonyms.net, searching gas does not yield vapor, and searching vapor does not yield gas, though I know they are more synonomous than not.

Good luck,
Latexman

 

[BigInch]

25362's definition works very well for me.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[Compositepro]

I actually get into this discussion occasionally. First I need to say that those two words are used so commonly in different ways that there is no strictly correct definition. However, when a distinction is made between the two words it is that a gas is generally considered non-condensible for practical puposes, while a vapor is produced by vaporizing a liquid and is close to being in equilibrium with its liquid form (as in water vapor).

The distinction is important in whether or not you must consider the issue of condensation or evaporation when dealing with a material. The difference in behavior can be quite large.

 

[bchoate]

nickyloke

A variety of comments have been offered in this discussion. Consider the following observations:

Conventionally, "vapor" and "gas" are often used interchangeably. But there are differences when one begins to critically look at the two.

"Vapor" and "gas" both refer to a physical state that is not solid or liquid.

A vapor is the gaseous state of a substance that most often presents as a liquid at ambient or near ambient conditions.

More specifically, a vapor is a substance in the gas phase at a temperature less than it's critical temperature. Vapor can condense to liquid upon small changes in pressure or temperature. A vapor is often in equilibrium with the liquid phase.

Similarly, gas cannot be condensed to liquid without changes in both pressure and temperature. A gas is a substance that at normal temperatures is above it's critical temperature. Liquid does not exist at these conditions. Special equipment and conditions are needed to liquify a gas.

 

[mbeychok]

.
At the risk of being redundant, I repeat unequivocally that the words gas and vapor are synonyms. They both name the phase of matter which is not solid and not liquid.

Read the given references in this article in the online Citizendium encyclopedia:

Click here ==> Gas

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[bchoate]

nlkw

mbeychok's comments are not disputed. At a number of levels "vapor" and "gas" are synonyms and engineers loosely use the terms interchangeably. They both refer to a phase of matter not solid or liquid as stated by more than one responder. However, some differences are described at higher levels of scientific inquiry. Some of these I listed in my previous post. These differences do not detract from the use of the terms as near synonyms. At the risk of appearing argumentative, I have viewed the terms as somewhat different during my 30+ year career. Although some may say that they use them as synonyms, in reality they probably do so inconsistently. The following example perhaps illustrates why I think this may be so.

Most us use "gas" when we talk about such things as nitrogen, oxygen, helium, carbon dioxide, nitrogen dioxide, etc. I dare say you have seldom used "vapor" when you spoke of these or even air. Yet, these gases could contain water, methanol, or hexane vapor and we would speak of these as vapor. In distillation systems we speak of methanol, water, acetone, or other distillates as vapors. Although both are the gas phase of matter, we often make a distinction. Perhaps we use vapor when speaking of materials in the gas phase that can be condensed to a liquid at ambient conditions. Those materials that we seldom see in any phase other than gas we call "gases". If these materials are processed in the appropriate temperature and pressure ranges in specialized equipment, the other phases may be seen.
I don't think anyone is disagreeing with "vapor" and "gas" as near synonyms. Nonetheless in practice I believe most of us make a distinction whether we realize it or not.

All the responders have contributed something to the discussion. However we individually use the terms, most of us have a fair understanding of what is being discussed. Each of us finds an understanding of the terms that is reasonable to us. I believe we can find consensus in that.

 

[nlkw]

Thanks for all the useful responses, especially to bchoate and 25362. I agree that there are varying levels of technical definition that we can get into, but staying with my original intent of collating ideas for the non-technical reader, I am inclined to exclude the complicated stuff.

I am looking for an easy to grasp illustration to use in my note and something that is applicable to fluid flow and measurement in the operating plant environment. Our training manual's target audience will most likely be plant operators or technicians, whom I assume prefer 'everyday hands-on' examples rather than engineering theory.

 

[jmw]

I think we should consider the relevance here to the posters industry, flow measurement.

In flow measurement a gas flowmeter might be vulnerable to condensation caused by pressure changes and here the distinction between a gas and a vapour based on 25362's definition might be considered appropriate.

We might also consider that we have situations with natural gas flow measurement where to measure the relative density a pressure let down system is sometimes required to prevent condensation which will mess up the density measurement.


JMW

http://www.ViscoAnalyser.com

 

[sheiko]

Another interesting point that you can highlight is that, unlike liquids and solids that are states that can have different phases (immiscible liquids constitues two phases for example), the gas (or vapor) state always has a unique phase (if you mix gases of different natures, it still remain one phase).
BTW, a phase is a state of matter in wich all properties are the same in every part of that matter.

"We don't believe things because they are true, things are true because we believe them."