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What is the difference between vapour and steam? These two terms seemingly are similar but have different meanings. trying to define these two gets us a similar definition which is not actually true I tried to think over it but could not.

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Depends on which meaning of steam you use; and what you're talking about with vapour.

In general, a vapour refers to the gas phase of a substance below critical temperature. This means that if pressure changes, the gas can become a liquid without a required temperature change. This can apply to any substance.

Steam in general refers to the gas phase of water. Steam can be the same as water vapour in some contexts. In more casual usage, steam can refer to the mixture of vapour and aerosol liquid water droplets suspended in the vapour. The "steam" you see rising off boiling water for example, is the second type. What you see is actually condensed liquid water; as water vapour itself is not visible to the naked eye.

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Steam is always a vapour. The viceversa is not true. Steam is the vapour obtained by heating a substance to its boiling point and as such is "hot", or better at boiling T. Vapour is the gas phases in equilibrium with a substance, or the part of it which escapes in an open system.

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    $\begingroup$ I would really like to know why downvoting. This is correct and that's kindergarten behavior $\endgroup$
    – Alchimista
    Feb 17, 2018 at 9:42
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    $\begingroup$ Even more so when a basically the same answer is up voted (nothing against the latter, of course. I could up vote it too). $\endgroup$
    – Alchimista
    Feb 17, 2018 at 9:56
  • $\begingroup$ I am not a native English speaker but would think that "steam" always refers to H$_2$O. $\endgroup$
    – user137289
    Feb 17, 2018 at 13:53
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    $\begingroup$ Probably the original poster's native language does not make the distinction either. So this is to a large extent a question about usage in English. But in that language, I do not think that one can say that "ethanol vapour is (a) steam". $\endgroup$
    – user137289
    Feb 17, 2018 at 14:18
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    $\begingroup$ I agree that sometimes the down votes on this site are so mysterious... $\endgroup$
    – yaobin
    Oct 1, 2021 at 12:06

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