Steam is a separate category as aroma in hilchot Kashrus. The former, if hot, contacts a pot cover, that cover would take on the dairy or meat status of the food below it. However, when water is heated, only H2O departs from the solution, and the solids are left behind. What you would get on the pot cover would be pure H2O, by which logic the cover should remain the status it was before the steam. However, the rabbi’s did not rule this way. Assuming that Chazal were more astute than we are, why do you suppose was the reason, aside from the obvious one of building fences.

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    Welcome to Mi Yodeya. This is a great question. My understanding of this concept is that if the taste of meat or dairy is present then this is what causes the problem. Scientifcally, we know that steamed food has an odor whereas purely steamed water is odorless. (Yes, I know that a typical shvitz (steam room) smells but that's prob. a combination of people's sweat, pipe smell and other items.) So, that indicates that the steam has elements of meat or dairy in it. When the steam condenses on the pot or wherever those particles of meat / dairy are on the lid or oven wall. (cont.)
    – DanF
    Aug 1 '19 at 19:17
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    Easy experiment - hold a clean spoon over the meat steam and let it gain some condensation. If you lick the spoon, you should taste meat.
    – DanF
    Aug 1 '19 at 19:19
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    Good question and +1 but halachic concepts are not always related to physics. Take absorption into a cooking pot. If one cooks meat in a pot, then cleans it, does food cooked a few hours later really taste of the meat? These are halachic constructs, which sometimes coincide with reality, sometimes act as fences as you write, and sometimes are guided by divine wisdom
    – mbloch
    Aug 1 '19 at 20:24
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    Indeed not all authorities accept your premise that steam is relevant and there is no clear Talmudic source for it
    – Double AA
    Aug 16 '19 at 17:01
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    I disagree with your premise that only H2O goes up. There are lots of flavor molecules that come along as well as small physical particulates that are carried along in the resulting gas flow. This is why steam doesn't smell like plain water: it's not.
    – Double AA
    Aug 16 '19 at 17:02

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