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Shower water

  1. Mar 10, 2005 #1

    Pengwuino

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    So why exactly do you get steam in the shower when you put hot water on? I assume the water isnt 100 C because im not jumping out of hte shower and yelling for my life so why is there steam?
     
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  3. Mar 10, 2005 #2

    Andrew Mason

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    You get steam in a sauna. You get warm air with high humidity in a shower.

    The warm air surrounding a jet of hot water can hold more moisture than the cooler air further away from the jet. When that warm moist air cools, it condenses and looks just like steam does when it condenses. In both cases, what you see is water vapour turning to water droplets due to condensation from cooling.

    AM
     
  4. Mar 10, 2005 #3

    HallsofIvy

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    Actually, you don't get steam in a sauna either.

    You cannot see steam- what you see is "water vapor" not steam!
     
  5. Mar 10, 2005 #4

    Andrew Mason

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    Well, you get water vapour at greater than 100 deg. C in a sauna, which I think is the definition of steam. Those hot rocks are hotter than 100 deg. C. But you are right, as I also pointed out, that what you see is condensed vapour, or water, not the vapour itself.

    AM
     
  6. Mar 10, 2005 #5

    russ_watters

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    The other half of the explanation is that you don't need water at 100C for water to evaporate. Its just that the hotter it is, the faster it evaporates.

    Also, as implied, the word "steam" is a little ambiguous and not really useful in this context because of the confusion it generates. Really, gaseous H2O at any temperature/pressure is "steam". But it can/does exist at a wide variety of temperatures and pressures. And water doesn't need to be boiling to generate it (though the specific molecules of steam have that energy level).

    edit: So what is so special about 100C? 100C is just the temperature at which a quantity of water will spontaneously turn to steam. Evaporation ("boiling" below boiling temp) occurs because individual atoms get "knocked off" the surface of the liquid water.
     
    Last edited: Mar 10, 2005
  7. Mar 10, 2005 #6

    chroot

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    Actually, Halls, you're wrong too. You cannot see either steam nor water vapor. Both are colorless.

    What you're seeing in the shower is many microscopic droplets of water suspended in air -- the same concept as a cloud. You don't need high temperatures; you just need supersaturated air. In fact, the shower is "steamier" on cold mornings because the vapor pressure of water is lower at low temperatures, so the water condenses into droplets more readily.

    - Warren
     
  8. Mar 11, 2005 #7
    does dry ice class as steam???
     
  9. Mar 11, 2005 #8

    cepheid

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    Dry ice is frozen CO2.

    As for the steam issue: can somebody clarify this please? It's still vague. I know that water can exist in all three phases at once, under the right conditions. I know that at room temperature, there is always a certain amount of moisture in the air (humidity), the percentage of which depends on the conditions. I believe that higher temperatures favour higher humidity (supported by what Chroot said about lower temperatures and lower humidity -- more of the moisture in the air condenses). I understand that water will spontaneously change to the gaseous phase if the vapour pressure of the water equals/exceeds atmospheric pressure. But what is this "vapour pressure" referring to? The pressure of little bit of gaseous H2O above the surface of whatever liquid water we're talking about, right? I don't know...all of the terminology is confusing. I guess I'm asking, are these distinct? --

    1. steam

    2. water vapour

    3. the "cloud" of condensed water droplets formed around microscopic particles in the air (they're called hygroscopic particles, right?)

    Or are two or more of them equivalent? Thanks. What exactly are 1 and 2, if not gaseous H2O? Thanks.
     
  10. Mar 12, 2005 #9

    Andrew Mason

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    I think you are referring to the cloud that you see around dry ice (solid CO2). That is caused by water vapour in the air condensing and forming suspended water droplets, which appear as fog. You don't see condensed CO2. You can make alot of fog by pouring hot water on dry ice.

    AM
     
  11. Mar 12, 2005 #10

    Andrew Mason

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    Right. So long as there is water around, there will be water vapour in the air. It contributes to the air pressure (ie. P = NRT where N is the number of water molecules/unit volume).

    1. and 2. Steam is not a precise term. Steam usually refers to water vapour at greater than 100 deg. C. produced by boiling water. Water will evaporate at less than 100 deg, of course, but one doesn't ususally refer to that water vapour as steam. For example, on a humid day we don't say that the air is full of steam.

    3 the cloud of condensed water droplets is NOT steam or water vapour. It is liquid water in fine droplet form. I don't know if there is a name for them.

    AM
     
  12. Mar 12, 2005 #11

    cepheid

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    Hmm, for what you said about #3, I agree that there doesn't seem to be specific name for them, although I guess clouds, mist, fog, etc. are all manifestations of that phenomenon right? (Although high altitude clouds can be made up of ice crystals and not water droplets, which makes sense to me, since it's damn cold up there.)

    I understand what you said about #2 since a vapour is just "the gaseous state of a substance that is normally in a liquid or solid state" (one definition I've seen, anyway). So in that case, water vapour is just a name for gaseous H2O, case closed.

    I agree that steam is an everyday term and is therefore not very precise. But I don't really understand why people here insist on saying that steam is synonymous with water vapour, that which happens to be produced by water boiling at 100 degrees celsius. They therefore go on to claim that since water vapour is colourless, so is steam, and so we can't see it. That statement seems a bit silly to me. Think of the most common contexts for the description: steam. A kettle, and the original topic: the stuff that fills the room when you take a shower. If steam is just water vapour, which cannot be seen, then what the heck is that stuff coming out of the kettle that everyone seems to be calling steam? Do you see my point? "Steam" seems to refer to what you can see in both of those contexts, i.e. steam is the "cloud" of condensed water droplets formed when water vapour at 100 C from boiling water hits the colder surroundings. So, doesn't it make more sense to classify steam as a #3 than a #2? Just wondering...

    Either that or AM's definition of steam is strictly correct, and the whole world's use of the word steam on an everyday basis is just erroneous. :rolleyes:
     
    Last edited: Mar 12, 2005
  13. Mar 12, 2005 #12

    chroot

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    cepheid,

    Many laypeople misuse words that have precise meanings in science.

    Steam refers to water in its gaseous phase. The term 'vapor,' as applied to all substances, refers to a portion of the substance in gaseous phase, even though the substance is nominally liquid or solid under those conditions.

    The stuff coming out of the kettle -- the visible stuff -- is just microscopic water droplets suspended in air. The stuff floating out of the shower is the same stuff.

    And yes, cepheid, you're right -- people misuse words like "steam" and "acceleration" and "force" allllll the time.

    - Warren
     
  14. Mar 12, 2005 #13

    Janitor

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    This isn't worth me started a new thread for, but I will throw it in here if you don't mind. A couple of weeks ago the local talk radio host asked if anybody "out there" knows why, when you open your dishwasher after it it finished cleaning dishes, the ceramic dishes are dry and the plastic dishes are beaded with water. One person called in to say that it is because the ceramic items have higher heat capacity, and therefore they retain a high temperature longer, such that as the moist air in the washer cools down, they don't condense it. Another caller said it is because plastic is porous, and ceramic isn't. The pores hold water.

    I like the first explanation better.
     
  15. Mar 12, 2005 #14

    Andrew Mason

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    It is not that the world's use is erroneous. It is just that in physics and engineering, steam has a particular importance and usefulness - it is a gas that has a high heat capacity so it can be used to do alot of work at high temperatures.

    If you defined steam as the stuff that you see (which, as I said is actually liquid water, not vapour) you could not get steam to a temperature greater than 100 deg. C (at atmospheric pressure).

    AM
     
  16. Mar 12, 2005 #15
    if you think of the density of these different forms of water, steam, as you can see it and feel it much easier than vapour, and apparantly it's actually liquid, is denser than vapour.

    list of densities:
    solid - iron
    plastics
    foams
    jelly
    gel/cream
    thick liquid
    liquid - water
    dense gas - steam, smoke, smog, fog, mist
    gas - water vapour
    plasma
     
    Last edited: Mar 13, 2005
  17. Mar 12, 2005 #16

    cepheid

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    Ok, thanks chroot and AM for clearing it up. That was a useful example too...steam doing work. I guess I forgot about steam engines and turbines.
     
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