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What is the difference between vapour and gas?
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Mark Cascella
What is the difference between vapour and gas?
I'm surprised the OED has such a strict definition for gas. I could not find a strict definition in the IUPAC color books (certainly not in the gold book). Presumably these words are in such common use that their definition is assumed understood or easily found. The analytical compendium (orange book) and physical chemistry book (green book) mention vapour (or vapor) in entries for the meaning of the abbreviation g:
g gas or vapour
The green book (physical chemistry) also has an entry for vapour pressure and for the abbreviation vap:
vaporization, evaporation (liquid→gas) vap
and definitions for a variety of acronyms that include the word vapor.
There is no suggestion there that gas should be restricted to the description of substances in their supercritical phase.
Curiously the IUPAC books do not adopt a uniform spelling convention. Some choose the spelling vapour, others vapor.
In common usage, a vapor is a gaseous substance emanating from a condensed phase. The scientific meaning is roughly consistent with common use. Vapor implies the existence of a condensed phase that is the source or destination of the gas, or with which the gas may be in equilibrium; while gas does not make such an assumption.
The origin of vapor is presumably Latin with earlier roots in Ancient Greek.
The origin of the word gas appears to be the Greek word chaos by way of Dutch:
Coined by chemist Van Helmont. Perhaps inspired by geest (“breath, vapour, spirit”) or by chaos (“chaos”), from Ancient Greek χάος (kháos, “chasm, void”).
I'm surprised the OED has such a strict definition for gas. I could not find a strict definition in the IUPAC color books (certainly not in the gold book). Presumably these words are in such common use that their definition is assumed understood or easily found. The analytical compendium (orange book) and physical chemistry book (green book) mention vapour (or vapor) in entries for the meaning of the abbreviation g:
g gas or vapour
The green book (physical chemistry) also has an entry for vapour pressure and for the abbreviation vap:
vaporization, evaporation (liquid→gas) vap
and definitions for a variety of acronyms that include the word vapor.
There is no suggestion there that gas should be restricted to the description of substances in their supercritical phase.
Curiously the IUPAC books do not adopt a uniform spelling convention. Some choose the spelling vapour, others vapor.
In common usage, a vapor is a gaseous substance emanating from a condensed phase. The scientific meaning is roughly consistent with common use. Vapor implies the existence of a condensed phase that is the source or destination of the gas, or with which the gas may be in equilibrium; while gas does not make such an assumption.
The origin of vapor is presumably Latin with earlier roots in Ancient Greek.
The origin of the word gas appears to be the Greek word chaos by way of Dutch:
Coined by chemist Van Helmont. Perhaps inspired by geest (“breath, vapour, spirit”) or by chaos (“chaos”), from Ancient Greek χάος (kháos, “chasm, void”).
Vapor (or steam for H2O) can be used for the gaseous form of a particular substance which at normal T and P are solid or liquid. They are all gases although not always ideal (steam is certainly not ideal below its critical T).
Vapor (or steam for H2O) can be used for the gaseous form of a particular substance which at normal T and P are solid or liquid. They are all gases although not always ideal (steam is certainly not ideal below its critical T).
In college, I had a thermodynamics teacher who was awesome. He had a way of explaining things that were accurate and easy to understand. He explained this difference to us this way:
A gas will not condense into a liquid with an isothermal compression. (i.e. an Ideal Gas)
A vapor will form liquid when isothermally compressed.
His example:
When you boil water, you get vapor. Hold a spoon over the kettle and it will get water droplets on it.
Steam is heated to a much higher temperature. "If you hold a spoon in from of a steam jet, there will be no condensation. Although, I suggest you not do that because you will be burned." (He tried to have a sense of humor.)
In college, I had a thermodynamics teacher who was awesome. He had a way of explaining things that were accurate and easy to understand. He explained this difference to us this way:
A gas will not condense into a liquid with an isothermal compression. (i.e. an Ideal Gas)
A vapor will form liquid when isothermally compressed.
His example:When you boil water, you get vapor. Hold a spoon over the kettle and it will get water droplets on it.
Steam is heated to a much higher temperature. "If you hold a spoon in from of a steam jet, there will be no condensation. Although, I suggest you not do that because you will be burned." (He tried to have a sense of humor.)
Back when I was taking thermo (and when I was a practicing Chemical Engineer), the distinction between gaseous water and water vapor was similar. Dry steam (More
Isothermal compression seems like a reasonable rule-of-thumb, though itd seem a bit off to me. For example, we wouldnt tend to say that a nebula is composed of vapor, regardless of its temperature. Isothermal-compression points in the right direction, but taken literally it would over-extend to near-vacuum conditions.More
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Vapor is a much older word alluding to dampness and it was not coined by scientists. It is in use since the 1300s. The actual meaning of meaning of vapor is "Matter in the form of a steamy or imperceptible exhalation; esp. the form into which liquids are naturally converted by the action of a sufficient degree of heat. This is the original 13th century meaning of vapor. The German equivalent of vapor is Dampf (which is relatively clearer in terms of feeling- imagine entering a basement which is damp from water vapor). Gas is relatively a new word as per the unabridged Oxford English Dictionary, and it entered English in the 1600s.
In fact, the OED mentions the distinction "Gas: A substance in a state in which it expands freely to fill the whole of a container, having no fixed shape (unlike a solid) and no fixed volume (unlike a liquid); spec. (distinguished from a vapour) such a substance above a critical temperature such that it cannot be liquefied by the application of pressure alone"
You can also think of the general meanings of vapor in scientific usage:
A substance which is a liquid at room temperature or under ambient conditions, and if it is heated enough so that the liquid evaporates (same root of word), you can call that as vapor of that substance.
Examples: Mercury is a liquid metal, when it is heated enough in a an electric bulb, it becomes a gas, hence the name mercury vapor lamp. It is not called a mercury gas lamp.
It is not necessary that heat be involved. You will find the usage "cold vapor method for Hg analysis" again for mercury which means that Hg was a liquid in a sample but by blowing a gas over it, it has been evaporated.
The recent meaning of vapor from OED, "Vapor: In modern scientific use, a fluid that fills a space like a gas but, being below its critical temperature, can be liquefied by pressure alone."
This is clear enough. Now apply this distinction to your phase diagram and notice the position of the critical point. The vapor "state" is below the critical point.
Vapor is a much older word alluding to dampness and it was not coined by scientists. It is in use since the 1300s. The actual meaning of meaning of vapor is "Matter in the form of a steamy or imperceptible exhalation; esp. the form into which liquids are naturally converted by the action of a sufficient degree of heat. This is the original 13th century meaning of vapor. The German equivalent of vapor is Dampf (which is relatively clearer in terms of feeling- imagine entering a basement which is damp from water vapor). Gas is relatively a new word as per the unabridged Oxford English Dictionary, and it entered English in the 1600s.
In fact, the OED mentions the distinction "Gas: A substance in a state in which it expands freely to fill the whole of a container, having no fixed shape (unlike a solid) and no fixed volume (unlike a liquid); spec. (distinguished from a vapour) such a substance above a critical temperature such that it cannot be liquefied by the application of pressure alone"
You can also think of the general meanings of vapor in scientific usage:
A substance which is a liquid at room temperature or under ambient conditions, and if it is heated enough so that the liquid evaporates (same root of word), you can call that as vapor of that substance.
Examples: Mercury is a liquid metal, when it is heated enough in a an electric bulb, it becomes a gas, hence the name mercury vapor lamp. It is not called a mercury gas lamp.
It is not necessary that heat be involved. You will find the usage "cold vapor method for Hg analysis" again for mercury which means that Hg was a liquid in a sample but by blowing a gas over it, it has been evaporated.
The recent meaning of vapor from OED, "Vapor: In modern scientific use, a fluid that fills a space like a gas but, being below its critical temperature, can be liquefied by pressure alone."
This is clear enough. Now apply this distinction to your phase diagram and notice the position of the critical point. The vapor "state" is below the critical point.
IMHO, usage of the term vapour is context dependent and is used for gas in context of liquid evaporation, condensation or equilibrium. So atmospheric CO2 is not considered vapour, even if it is below its critical point. OTOH, atmospheric gaseous water is considerer vapour, as out is related to water evaporation and condensation.More
You are right, again, I think the reason is that carbon dioxide does not exist in liquid phase under ambient conditions. The context is always important.More
@Goarkz As far as I can tell their distinction is less useful but more precise: they only label the region where there is no distinction between gas and liquid at any pressure. My definition is closer to normal usage which considers both pressure and temperature.More
Well, perhaps because nobody usually works with liquified carbon dioxide and liquid carbon dioxide does not exist under ambient conditions. But imagine if you had a transparent chamber full of liquid CO2, below its critical point, it would be fine to call it CO2 vapor in equilibrium with liquid CO2. Google Scholar shows the usage of carbon dioxide vapor in equilibrium with its liquid.More
I'm surprised the OED has such a strict definition for gas. I could not find a strict definition in the IUPAC color books (certainly not in the gold book). Presumably these words are in such common use that their definition is assumed understood or easily found. The analytical compendium (orange book) and physical chemistry book (green book) mention vapour (or vapor) in entries for the meaning of the abbreviation g:
The green book (physical chemistry) also has an entry for vapour pressure and for the abbreviation vap:
and definitions for a variety of acronyms that include the word vapor.
There is no suggestion there that gas should be restricted to the description of substances in their supercritical phase.
Curiously the IUPAC books do not adopt a uniform spelling convention. Some choose the spelling vapour, others vapor.
In common usage, a vapor is a gaseous substance emanating from a condensed phase. The scientific meaning is roughly consistent with common use. Vapor implies the existence of a condensed phase that is the source or destination of the gas, or with which the gas may be in equilibrium; while gas does not make such an assumption.
The origin of vapor is presumably Latin with earlier roots in Ancient Greek.
The origin of the word gas appears to be the Greek word chaos by way of Dutch:
I'm surprised the OED has such a strict definition for gas. I could not find a strict definition in the IUPAC color books (certainly not in the gold book). Presumably these words are in such common use that their definition is assumed understood or easily found. The analytical compendium (orange book) and physical chemistry book (green book) mention vapour (or vapor) in entries for the meaning of the abbreviation g:
The green book (physical chemistry) also has an entry for vapour pressure and for the abbreviation vap:
and definitions for a variety of acronyms that include the word vapor.
There is no suggestion there that gas should be restricted to the description of substances in their supercritical phase.
Curiously the IUPAC books do not adopt a uniform spelling convention. Some choose the spelling vapour, others vapor.
In common usage, a vapor is a gaseous substance emanating from a condensed phase. The scientific meaning is roughly consistent with common use. Vapor implies the existence of a condensed phase that is the source or destination of the gas, or with which the gas may be in equilibrium; while gas does not make such an assumption.
The origin of vapor is presumably Latin with earlier roots in Ancient Greek.
The origin of the word gas appears to be the Greek word chaos by way of Dutch:
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Vapor (or steam for H2O) can be used for the gaseous form of a particular substance which at normal T and P are solid or liquid. They are all gases although not always ideal (steam is certainly not ideal below its critical T).
Vapor (or steam for H2O) can be used for the gaseous form of a particular substance which at normal T and P are solid or liquid. They are all gases although not always ideal (steam is certainly not ideal below its critical T).
More
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In college, I had a thermodynamics teacher who was awesome. He had a way of explaining things that were accurate and easy to understand. He explained this difference to us this way:
A gas will not condense into a liquid with an isothermal compression. (i.e. an Ideal Gas)
A vapor will form liquid when isothermally compressed.
His example: When you boil water, you get vapor. Hold a spoon over the kettle and it will get water droplets on it.
Steam is heated to a much higher temperature. "If you hold a spoon in from of a steam jet, there will be no condensation. Although, I suggest you not do that because you will be burned." (He tried to have a sense of humor.)
In college, I had a thermodynamics teacher who was awesome. He had a way of explaining things that were accurate and easy to understand. He explained this difference to us this way:
A gas will not condense into a liquid with an isothermal compression. (i.e. an Ideal Gas)
A vapor will form liquid when isothermally compressed.
His example:When you boil water, you get vapor. Hold a spoon over the kettle and it will get water droplets on it.
Steam is heated to a much higher temperature. "If you hold a spoon in from of a steam jet, there will be no condensation. Although, I suggest you not do that because you will be burned." (He tried to have a sense of humor.)
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Vapor is a much older word alluding to dampness and it was not coined by scientists. It is in use since the 1300s. The actual meaning of meaning of vapor is "Matter in the form of a steamy or imperceptible exhalation; esp. the form into which liquids are naturally converted by the action of a sufficient degree of heat. This is the original 13th century meaning of vapor. The German equivalent of vapor is Dampf (which is relatively clearer in terms of feeling- imagine entering a basement which is damp from water vapor). Gas is relatively a new word as per the unabridged Oxford English Dictionary, and it entered English in the 1600s.
In fact, the OED mentions the distinction "Gas: A substance in a state in which it expands freely to fill the whole of a container, having no fixed shape (unlike a solid) and no fixed volume (unlike a liquid); spec. (distinguished from a vapour) such a substance above a critical temperature such that it cannot be liquefied by the application of pressure alone"
You can also think of the general meanings of vapor in scientific usage:
A substance which is a liquid at room temperature or under ambient conditions, and if it is heated enough so that the liquid evaporates (same root of word), you can call that as vapor of that substance.
Examples: Mercury is a liquid metal, when it is heated enough in a an electric bulb, it becomes a gas, hence the name mercury vapor lamp. It is not called a mercury gas lamp.
It is not necessary that heat be involved. You will find the usage "cold vapor method for Hg analysis" again for mercury which means that Hg was a liquid in a sample but by blowing a gas over it, it has been evaporated.
The recent meaning of vapor from OED, "Vapor: In modern scientific use, a fluid that fills a space like a gas but, being below its critical temperature, can be liquefied by pressure alone."
This is clear enough. Now apply this distinction to your phase diagram and notice the position of the critical point. The vapor "state" is below the critical point.
Vapor is a much older word alluding to dampness and it was not coined by scientists. It is in use since the 1300s. The actual meaning of meaning of vapor is "Matter in the form of a steamy or imperceptible exhalation; esp. the form into which liquids are naturally converted by the action of a sufficient degree of heat. This is the original 13th century meaning of vapor. The German equivalent of vapor is Dampf (which is relatively clearer in terms of feeling- imagine entering a basement which is damp from water vapor). Gas is relatively a new word as per the unabridged Oxford English Dictionary, and it entered English in the 1600s.
In fact, the OED mentions the distinction "Gas: A substance in a state in which it expands freely to fill the whole of a container, having no fixed shape (unlike a solid) and no fixed volume (unlike a liquid); spec. (distinguished from a vapour) such a substance above a critical temperature such that it cannot be liquefied by the application of pressure alone"
You can also think of the general meanings of vapor in scientific usage:
A substance which is a liquid at room temperature or under ambient conditions, and if it is heated enough so that the liquid evaporates (same root of word), you can call that as vapor of that substance.
Examples: Mercury is a liquid metal, when it is heated enough in a an electric bulb, it becomes a gas, hence the name mercury vapor lamp. It is not called a mercury gas lamp.
It is not necessary that heat be involved. You will find the usage "cold vapor method for Hg analysis" again for mercury which means that Hg was a liquid in a sample but by blowing a gas over it, it has been evaporated.
The recent meaning of vapor from OED, "Vapor: In modern scientific use, a fluid that fills a space like a gas but, being below its critical temperature, can be liquefied by pressure alone."
This is clear enough. Now apply this distinction to your phase diagram and notice the position of the critical point. The vapor "state" is below the critical point.
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tl;dr– "Gas" and "vapor" aren't mutually exclusive. Generally:
a gas is any material that'd fill a volume to its boundaries; and
a vapor is a gas-like material that's associated with a condensed-state transition.
It's a bit misleading for a state-diagram to label a region "vapor" in a manner that might imply that a vapor's not a gas.
State-diagrams should refer to "gas", not "vapor".
Definitions:
A gas is material that would fill any volume up to the boundaries, ignoring gravity and the like.
A vapor is a gas or gas-like material that's associated with a condensed-state transition.
These aren't alternatives; something can be a gas or/and a vapor. Examples of each combination:
tl;dr– "Gas" and "vapor" aren't mutually exclusive. Generally:
a gas is any material that'd fill a volume to its boundaries; and
a vapor is a gas-like material that's associated with a condensed-state transition.
It's a bit misleading for a state-diagram to label a region "vapor" in a manner that might imply that a vapor's not a gas.
State-diagrams should refer to "gas", not "vapor".
Definitions:
A gas is material that would fill any volume up to the boundaries, ignoring gravity and the like.
A vapor is a gas or gas-like material that's associated with a condensed-state transition.
These aren't alternatives; something can be a gas or/and a vapor. Examples of each combination:
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