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Ask a physicist anything. (6)

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Wiccan_Child

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Lillen said:
And another question: Why does ocean water evaporate under the degree of 100 celsius?
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This works with any water. A water molecule on the surface will evaporate when it hits 100°C. The average temperature could be about 25°C, but the jostling and colliding of molecules can make an individual molecule's temperature soar well above that average - meaning it can boil.

This is also why puddles evaporate on their own.
 
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Mr. Pedantic

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You say that you know we are lying because you cannot understand us. You also say that you would know if we were telling the truth because then you would be able to understand us. But how do you know that if we told you something you did understand, you would know it to be the truth? That is to say, how do you know that all the statements you understand are truths, and all the statements you don't are lies?
 
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Chalnoth

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Lillen said:
I do not wish to change topic in other peoples threads... I stick with the last question for WC claim that puddles could evaporate - How?
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The basic mechanism is that the molecules of water in the puddle are continually knocking into each other. Randomly, some number of molecules are periodically given enough energy to leave the puddle of water entirely (i.e., evaporate). As long as this happens faster than molecules of water from the air return to the puddle, it will, in time, evaporate to nothing.
 
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chris4243

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Lillen said:
And another question: Why does ocean water evaporate under the degree of 100 celsius?
Click to expand...

The water evaporates because the H2O vapor pressure of the air near the water is greater than the H2O vapor pressure of the air far away from the water, and the water molecules are constantly diffusing away. In a closed container of water the H2O vapor pressure would be the same throughout, and molecules of water would evaporate and condense at the same rate (though they would still leave the liquid and leave the air, only at the same rate so that overall nothing seems to happen).

The vapor pressure increases as temperature increases, and eventually if it gets hot enough the vapor pressure equals the atmospheric pressure, and bubbles of vapor start forming throughout the liquid. We call this boiling, and the temperature at which it happens the boiling point. Note that the ocean would not boil at 100 C, because the addition of salt raises the boiling point slightly. Likewise, if you lower the atmospheric pressure the water will boil at a lower temperature because it doesn't have to get so hot for the vapor pressure to equal the atmospheric pressure -- hence the difficulties of high altitude cooking, and the purpose of pressure cookers.
 
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Mr. Pedantic

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Something being 'lighter than air' doesn't really mean anything. A good rule of thumb is that the more massive a particle, the more significant gravity is, compared to other forces (like electromagnetism, strong/weak nuclear forces). Conversely, the smaller things are, the less affected they are by gravity. You can see this somewhat for insects, but you really see it for individual molecules. At that scale, gravity is almost irrelevant on a local scale. So steam will go in any which way.


In normal air, steam normally rises because the steam and the boiling water heats the air, and hot air rises because it is less dense than more dense air.
 
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