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Why does hot water are white?

  1. Apr 26, 2010 #1
    Hi everyone
    Today, me and my brother made pancakes for breakfast... after we finished to eat I started to clean up the skillet... to made the cleaning action easier I filled the skillet with hot water (which were white), then I left the kitchen for few minutes. When I came back I found out that the water became clear...
    My question is why does hot water are white and how (and why) they became clear...

    Sorry if its not the right forum, and sorry for the spelling and grammar mistakes, I'm not a native speaker...
     
  2. jcsd
  3. Apr 26, 2010 #2
    Air bubbles in the water makes it transluent or opaque. Over time the bubbles reach the surface and escape the body of water, making it transparent.

    transluent - you can see diffuse shapes throguh it, but no details
    opaque - to light goes through. Completely unclear.
    transparent - clear
     
    Last edited: Apr 26, 2010
  4. Apr 26, 2010 #3
    Why does hot water have more air bubbles than cold water?
     
  5. Apr 26, 2010 #4
    Not really sure. By leaving the tap on in front of a monochrome background, I see that the bubbles appear immediately after the water leaves the tap.The reason the warm water takes up air in free fall might be that the surface of the water is "bulkier" and thus has more pockets where air bubbles can enter. Just a guess. I'm not really that experienced with fluid mechanics.
     
  6. Apr 26, 2010 #5

    Ygggdrasil

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    Gasses are less soluble in hot liquids than in cold liquids, though I'm not sure if this is involved in the process.
     
  7. Apr 26, 2010 #6
    the bubbles could dissolve more in the cold water... as well as some of the residue from pancakes dissolving in the water with time
     
  8. Apr 26, 2010 #7
    I wouldn't call it a solution in this case. A better analogy might be a colloid of a solid dispersed in a liquid, only it is a gas istead of a solid.
     
  9. Apr 26, 2010 #8
    The bubbles are caused by the water vaporizing in the flow stream.
     
  10. Apr 26, 2010 #9

    Borek

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    That would mean boiling hot water is necessary, while water can be white - full of small bubbles - even when its temperature is much lower. My guess is that Ygg is right - when water is being warmed dissolved gasses start to leave the solution - hence lots of small bubbles, small enough that the water looks white and they need some time to get up to the water surface where they disappear. Additionally when water leaves the tap its pressure goes down - difference is probably not large, but can be high enough to start "bubblification".
     
  11. Apr 26, 2010 #10
    Not that I'm necessarily disagree with you, but how exactly do such large amounts of air get dissolved into the water? Its easy to see how oxygen will dissolve in standing water such as a pond or an ocean but what about our water system? I always thought tap water mostly remained in tanks and pipes, i.e. not exposed to air.

    On another note, water doesn't have to reach 100'C in order to vaporize. You can vaporize water quite easily at room temperature using siphon. Thing such as cavitation are a big problem pumps and turbo machinery. I'm not 100% on this but I believe if you get water near boiling, maybe around 80'C, and throw in some turbulence and high velocities you could probably reach low enough pressures in those eddys that the water will vaporize. One test you can do is run hot water out of your tap slowly (laminar flow) and the quickly (turbulent flow) and look at the difference.

    I think the word you are looking for is deaeration.
     
  12. Apr 26, 2010 #11
    Another thing I forgot to mention is that when you run hot water out of the faucet you see steam, aka water vapor. The water isn't at boiling but you still have water vaporizing meaning you that if you have water vapor existing in the air then its probably going to exist in the liquid water as well.
     
  13. Apr 27, 2010 #12

    Borek

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    Water was already saturated with gases before it entered the pipes. Seince then it was slightly pressurised and it had no chance to degass.

    But that's exactly difference between vaporisation and boiling - liquids always vaporize on the surface, no matter what their temperature is, but to vaporise inside - that's definition of boiling. So as long as you are below boiling point, there should be no vapor bubbles inside. Unless they are effect of cavitation that you mentioned. However, cavitation is forced in areas of very low pressure, and vapor bubbles - when they flow into areas of normal pressure - will immediately collapse back, as they are thermodynamically unstable. Air bubbles are stable.
     
  14. Apr 27, 2010 #13

    Mapes

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    I agree with espen180 and Borek: the white appearance of the water is from air bubbles in suspension. Once thing nobody's mentioned is that water from sink faucets is often deliberately aerated through a couple of filters to give the appearance of a large flow while reducing water use.

    Topher925, the reason these can't be vapor bubbles is that these aren't stable at room temperature and pressure. Even if some kind of plumbing disaster caused cavitation-inducing turbulence upstream, these bubbles would immediately collapse at atmospheric pressure. And as Borek mentioned, all materials have a vapor pressure, but that doesn't mean they nucleate bubbles below their boiling temperature.
     
  15. Apr 27, 2010 #14
    I did not know this. This morning I tried running the hot water into a glass to see this bubble formation but there was nothing at all. But then I remembered that we have well water and not water from a treatment plant. So I'm guessing there are no aeraters in the system.


    Anyway, my mistake, I guess it is deaeration. Found a good link on the subject: http://www.engineeringtoolbox.com/air-solubility-water-d_639.html
     
    Last edited: Apr 27, 2010
  16. Apr 27, 2010 #15

    Borg

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    Isn't the aerator just the wire mesh screen on the faucet? Maybe you don't have one.

    Edit. I just saw Mapes post about filters. I was looking for the word screen.
     
  17. Apr 27, 2010 #16

    Mapes

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    I was thinking "screens" but my fingers wrote "filters." :smile:
     
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