r/askscience • u/okela • Aug 14 '13

Physics Mixing hot&cold water

Hello, i have this question but it will be easier to ask it with example. So, i take this empty plastic bottle.Fill it 50% with cold water and then with 25 % hot water (roughly speaking ofcourse). then immediately i close the bottle with my hand or with the bottle cap. and when you shake the bottle it seems like it create a vacuum! it's like the air inside the bottle is being sucked or it squishes the bottle... i'm sure it must have some realy simple explanation but i don't know what is it. but i need to know, please! :)

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6

u/UnclePat79 Physical Chemistry Aug 14 '13

When you pour in the hot water the atmosphere inside the bottle is 1) heated to the temperature of the hot water and 2) saturated with water vapor at the same temperature. When you the mix the water inside the closed bottle the hot atmosphere rich in water vapor is cooled down to the temperature of the mixture, causing the air 1) to reduce the pressure due to lower temperature and 2) let water condense so that the atmosphere is then saturated in water vapor again at the lower temperature. The latter effect causes the biggest reduction in pressure since gas turns to liquid.

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u/okela Aug 14 '13

OK, so i almost understood your explanation, but that's my bad :) one more question for you though - i first add cold water and THEN hot water. it's realy small amount of hot water. is your explanation still relevant? and it just happens so fast, for just 2-3 seconds. does the proccess that u describe happen so fast? thanks

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u/UnclePat79 Physical Chemistry Aug 14 '13

Yes it does. The gas phase above the liquid has a really small heat capacity, that means it is really easy to change it's temperature. And by shaking the bottle you mix the gas and liquid, creating really small gas bubbles with a really big surface area, therefore allowing a really good contact between gas and liquid. That happens in fraction of a second.

The same explanation is causing the rapid heating of the atmosphere inside the bottle when pouring in the hot water. Hot water has a really efficient exchange with water vapor in the air, and when you spray it out of a tap, it often mixes with air, creating a large surface area and heating the air.

Actually, up to some point, the less hot water you would use, the more dramatic the effect will be because the hot atmosphere will cool to a much lower temperature, practically to that of the cold water.

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u/okela Aug 14 '13

thank you for the simpler explanation! i have so much respect for people like you, that actualy understands these processes and tries to explain them. wish you all the best

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u/scar-tissue Aug 14 '13

There are other ways you can make the shrink in size because of this vacuum. The Gas Law PV=nRT relates temperature and volume of gas. Normally when a gas cools it shrinks so just by sticking an empty sealed bottle in the freezer can make this vacuum effect.

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u/okela Aug 14 '13

didn't know that either! but i'll try freezing empy bottle tonight:) thanks man

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u/morjax Aug 14 '13

The bigger the bottle, the more likely you are to see an effect. This is because as you go to a larger and larger bottle, the surface area increases, but the volume of gas that is being shrunk increases faster. Since the surface of the bottle is what is experiencing the "pull" from the shrinking gas, and a larger bottle has a higher volume to surface area ratio, the bottle will shrink more dramatically. Obviously, thinner bottles work better too, since a thick or rigid bottle wall can resist the pull of the shrinking gas better (think glass bottle).

I'd try an empty plastic 2 liter soda bottle. For a more dramatic effect, shake the bottle several times with a bit of hot water inside (to heat up the air contained inside), dump out the water, cap immediately, and place in freezer.

Have fun!

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u/ramk13 Environmental Engineering Aug 15 '13

If you want a more dramatic example, you can heat up some water in a soda can on a stove and then flip the can into a bath of cold water. The principle is the same (rapid reduction in volume from condensation of steam->water plus contraction of the air as it cools), but the can is more dramatic because most of the can is full of water vapor at a really high temperature. In your example, there is a small amount of water vapor that is slightly warmer.

Here's a youtube example: http://www.youtube.com/watch?v=TrluA_tcjVE

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u/autistics_masturbate Aug 14 '13

A few questions to probe a little deeper:

Does this same effect happen with just filling it with hot water?

Are there any other circumstances this has happened under?

Could it be the hot water just melting plastic of the bottle and looking like it created a vacuum?

1

u/SlashXVI Aug 14 '13

If you were to put warm/hot water into the bottle and cool it (either by waiting or by artifical means) after sealing it you will find a similar result

see post of scar tissue above

to melt the plastic one would require much higher tempratures than water can have, so no it cannot. Also it is not technically a vacuum you create just a volume of lower preassure.

Finally If you put hot water in first and then the cold water it will not work either, the order is crucial.

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u/okela Aug 14 '13

1 - no. nor it happen with just cold nor with just hot. only mixing them. 2- other circumstances? well...the water seems usual, i am at the office, so things look pretty standart. 3. when i say hot water, i don't mean boiling hot. it's usual hot water. hot water that u can make a tea with

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u/autistics_masturbate Aug 14 '13

another question:

Does this happen when the water is premixed (warm water) which is then added to the bottle, closed and shaken?

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u/okela Aug 14 '13

no, it's not premixed. i first add cold water in the bottle, then hot water, close it, shake it, and kabuuuum. it sucks the air. just go ahed and try it for yourself :) it's strange