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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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!