Short answer
Gas names a phase; vapor adds context about that gaseous matter’s relationship to a liquid or solid. The terms overlap: OpenStax describes the same molecules as gas when discussing their motion and as vapor when discussing equilibrium with liquid. Temperature matters to the situation, but these excerpts do not establish a temperature cutoff between the words. 1
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At a glance
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| Question | Gas | Vapor |
|---|---|---|
| What does the word emphasize? | Gaseous phase | Connection to a condensed phase |
| Can it describe molecules above a liquid? | Yes | Yes |
| Must liquid still be present? | Not in the heating example | Not in the heating example |
| Is it a separate state? | The phase label | No separate state shown |
These distinctions follow the sources’ usage, not a universal naming rule. 1 2
What each thing is
In these accounts, gas is the phase reached through vaporization or sublimation. Vapor is the gaseous portion discussed in relation to condensed matter. LibreTexts connects vapor pressure to equilibrium with condensed phases, so the relevant counterpart need not be liquid alone. 1 2
Key differences
The useful difference is emphasis, not a different molecular identity. Gas tells you which phase is being discussed. Vapor directs attention toward phase exchange or the gaseous portion’s relationship to condensed material. For an actual system, specify temperature and whether condensed material remains rather than expecting either word to supply those details. 1
How to tell them apart
A practical reading rule: interpret gas as the phase label, and look for evaporation, condensation, sublimation, or equilibrium when vapor appears. The limit is important: vapor alone does not prove that liquid is currently present. OpenStax still calls water vapor by that name after all the liquid has evaporated. 1
Where they overlap
A closed container holding liquid and its gaseous phase makes the overlap especially clear. At equilibrium, molecules continue moving between phases even though their amounts remain unchanged. Calling the upper portion gas describes its phase; calling it vapor highlights its exchange with the liquid. Both descriptions fit. 1
Edge cases
Water vapor at 120 °C is a useful counterexample to defining vapor as something that must currently sit above liquid. OpenStax’s example boils the water and then heats the resulting steam further. That temperature belongs to the example, not to a general boundary between gas and vapor. 1
Why the distinction exists
Vapor adds useful information when the subject is phase behavior. In a vapor-pressure discussion, the connection to condensed material and equilibrium is central; merely identifying the gaseous phase leaves that relationship unstated. The word is useful context, but it does not replace a description of the system’s conditions. 2
Common misconceptions
Do not treat gas and vapor as mutually exclusive states, or assume vapor necessarily means boiling is underway. LibreTexts explains escape into the gaseous phase through the range of individual molecular energies. Conversely, calling something gas does not, in these sources, establish that it cannot condense. 2
Examples
First, imagine a sealed container with liquid water remaining and equal rates of evaporation and condensation: its gaseous water is both gas-phase matter and vapor in equilibrium with liquid. Second, in OpenStax’s worked example, water becomes vapor at 120 °C after boiling and further heating; vapor does not imply remaining liquid. 1