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When I make pasta or potatoes, I dispose of the hot water (around 80-100°C) in a metal sink. Am I justified to first pour regular hot tap water on the sink to reduce the amount of steam generated, or is this pure superstition ? If it's not, what is the physical explanation ?

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  • $\begingroup$ I have no thermometer. Maybe it's (100° - ε). But the steam is there all the same. $\endgroup$
    – nmajoros
    Mar 2 '20 at 11:00
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    $\begingroup$ Why do you put hot tap water into the sink rather than cold tap water? $\endgroup$
    – PM 2Ring
    Mar 2 '20 at 14:16
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    $\begingroup$ @Sam The vapour pressure of water is almost half an atmosphere at 80C, so yes, there will be steam, of course. $\endgroup$
    – J...
    Mar 2 '20 at 19:53
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    $\begingroup$ @sam By “steam”, OP means the vapor that condenses above the hot water. This is standard usage in English and has been since the 17th century. In fact the scientific usage to mean the invisible gaseous phase of water is a much more recent development. $\endgroup$ Mar 2 '20 at 21:09
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    $\begingroup$ Just out of curiosity, why do you want to reduce the vapor given off? What is the purpose? $\endgroup$
    – Bob D
    Mar 2 '20 at 21:26
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By running hot water in the sink first and thus pre-heating the metal sink, you are increasing the amount of steam. The reason is explained in the other answers, the visible "steam" (not gaseous) consists of fluid water droplets that condensed from vapour that previously evaporated due to high vapour pressure due to high temperature of water. So if the sink is hot the water is cooled less and more of it evaporates. If you run cold water in the sink and keep the tap on, that's usually enough to completely stop such a wave of hot mist on pouring in hot water. It also prevents the banging noises that some sinks produce because of thermal expansion when pouring hot water into them. Neither heating nor cooling the sink is necessary or objectively useful before pouring in boiling water (the exception is some sinks for example in camping cars which aren't heat proof and you aren't allowed to put boiling water in them at all).

The other answers are fine from a standpoint of physical correctness, I just thought explitcitly pointing out that what you are doing is backwards made sense.

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If you fill the sink with colder water there will not be much steam, but if you just pour a layer of colder water it will not have much effect. The reason is that the mass of boiling water is much larger than the layer, so the temperature reduction will be much smaller than for mixing it with an about equal volume of colder water.

Typically boiling water will stop boiling nearly instantly when you take it off the stove: it is kept boiling by adding energy, and quickly goes below 100C due to energy loss due to steam. The remaining steaming is because it is still warm enough to have an appreciable vapour pressure, but going down to 80C about halves it. So even a fairly small temperature reduction can reduce the steaminess noticeably.

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  • $\begingroup$ I don't think 'filling' or putting a thin layer of average temperature water is what I'm doing. The sink is in metal, a good conductor, I'm bringing it to average, higher temperature. $\endgroup$
    – nmajoros
    Mar 2 '20 at 11:42
  • $\begingroup$ On the other hand, maybe I'm simply getting it wet, and it smooths it, wiping away nucleation sites, like a wet beer glass will make less foam when you pour beer in it. $\endgroup$
    – nmajoros
    Mar 2 '20 at 11:43
  • $\begingroup$ The heat capacity of modern sinks tend to be rather low. That it conducts heat well doesn't matter much if it becomes essentially the same temperature as the water. The steam above the surface is not due to boiling nucleation. $\endgroup$ Mar 2 '20 at 14:59
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Steam is water in the gaseous phase and is invisible. It occurs when water boils which, at one atmosphere, is a temperature of 100 C.

What you are seeing coming up from your sink is not steam, but water vapor that is condensing into liquid water droplets. You can see the same thing above a hot, but not boiling, cup of coffee. It is an accelerated form of evaporation of water at the surface of the liquid due to its high temperature. Evaporation occurs at the surface of the liquid even for liquids at room temperature, though the rate is slow. It is due to the escape of the higher kinetic energy water molecules that exist at the surface. The higher the temperature of the water, the greater the kinetic energy of the molecules at the surface and the greater the rate of evaporation. Then when the water vapor mixes with the cooler air above the surface it condenses into water droplets.

Hope this helps.

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    $\begingroup$ Am I confused, or does this not actually answer the OP at all? It seems to nitpick over steam/vapor (irrelevant to the actual question) and not address whether first running hot water will reduce the amount of steam that billows up when you pour boiling water into the sink. $\endgroup$ Mar 2 '20 at 21:17
  • $\begingroup$ @GregSchmit The OP didn't say "boiling" water was poured into the sink, and obviously thought steam was something you see. I simply wanted to make sure the OP understood the difference between water vapor and steam. $\endgroup$
    – Bob D
    Mar 2 '20 at 21:25
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    $\begingroup$ Is that what you got from my comment? I said boiling because it's a normal use of the English language to say that you're pouring boiling water when you pour "water which was just removed from the burner and just recently stopped boiling". I don't mind the clarification and explaining steam/vapor, but you are not comprehending what the OP's question was. The question was, rephrased: Call the hot stuff in the air when you pour hot water in the sink X. Does running the hot water first reduce the amount of X? $\endgroup$ Mar 2 '20 at 21:30
  • $\begingroup$ @GregSchmit This is a technical site, is it not. Lots of stuff in the "normal use of the English language" is not technically correct and when it is used incorrectly I believe it should be pointed out. In any case, why don't you post an answer of your own if you don't like mine. $\endgroup$
    – Bob D
    Mar 2 '20 at 21:42
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    $\begingroup$ Right, but correcting semantics while not answering the question makes for a poor answer IMO. I don't know enough about this subject to provide an answer (look at my rep, I mostly am on SO, not physics.SE). I just noticed this in the HNQ and I thought this was a surprisingly bad answer because you correct their semantics and ignore the actual question. $\endgroup$ Mar 2 '20 at 22:21

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