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For a while now I've noticed that if I take a cup of hot tea and pour it into two cups and leave it then both cups will cool faster than the single cup.

How is it that nature cools two cups of tea in parallel more quickly? It's still the same amount of tea but it gets cooled quicker if I have two cups.

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The tea cools mostly by evaporation - when you pour it into two cups you will have twice the surface area.

During evaporation, the fastest (hottest) water molecules escape the liquid, leaving on average a cooler liquid behind (when the richest man leaves the room, the average wealth in the room drops).

The "evaporation cools down tea" concept is well known by Indias Chai Wallahs - see for example this video. There are more spectacular examples but I could not locate one right now.

There is a secondary effect of heat capacity: when you pour tea into a cold cup, some of the heat in the tea is used to warm up the cup. Two cups to warm up = more heat extracted from the tea. But that is a one time effect. The evaporation keeps going.

One other reason why the chai wallah trick is so effective (and why blowing on your tea cools it more quickly): as water evaporates, it increases the partial vapor pressure right next to the liquid. If that vapor is not removed, the result is that evaporation (and cooling) slows down. The pouring trick ensures the vapor can escape easily - the liquid is always surrounded by fresh (somewhat dry) air. Note that even if you do this in highly humid air (relative humidity 95%), since the tea is hotter than the air it heats the local air which allows more vapor to go into it. But the rate of cooling will be greatest when the air is driest. Within limits, that is more important than how cold the air is.

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    $\begingroup$ To your tea comment, this video comes to mind: wimp.com/hottea (man throws the tea at high speed to cool it down). $\endgroup$ – schil227 Jul 26 '15 at 15:25
  • $\begingroup$ @schil227 thanks - that was the spectacular one I had trouble finding! $\endgroup$ – Floris Jul 26 '15 at 16:20
  • $\begingroup$ Or if you remember during the US cold snap last winter, the viral video trend for throwing boiling water in your own face? Well, the theory was that the water would freeze mid-air, and if you throw it downwind instead of upwind it sometimes has enough time before impact to do so ;-) $\endgroup$ – Steve Jessop Jul 26 '15 at 23:12
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    $\begingroup$ "One time effect" — not true, the glasses continuously act as heatsinks. $\endgroup$ – Elliot Gorokhovsky Jul 27 '15 at 1:24
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    $\begingroup$ @RenéG - once the container is at the same temperature as the tea, it will stop absorbing heat from it. After that, it will still act as a poor conductor of heat - but that is not a heat capacity effect, and much less significant than the evaporation. $\endgroup$ – Floris Jul 27 '15 at 2:28
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A certain amount of hot liquid shared among two glasses would have more of its surface exposed to air than the same amount of liquid in a single glass. That rate of cooling is higher if more of the liquid is exposed to a colder body. Air, in this case. (which generally is cooler than everyday hot liquids like tea.)

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    $\begingroup$ While this is important, evaporation cooling is much stronger in a case like this. Glass is quite thermally conductive, so there shouldn't be a big difference between one glass and two (you only increased the contact area by the surface area of the water liquid - the rest is the same with one glass). On the other hand, the evaporative area doubled - effectively also doubling the cooling rate (approximately). $\endgroup$ – Luaan Jul 27 '15 at 8:04
  • $\begingroup$ I'll keep that in mind. $\endgroup$ – Hritik Narayan Jul 27 '15 at 11:47
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Hritik already gave the correct answer. I would like to add a related fact: You can very nicely observe this phenomenon by the way nature has adopted to different temperatures. Antarctic penguins are usually much bigger than e.g. their Australian relatives.

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    $\begingroup$ This deserves some more clarification, in that the bigger animals have less surface in proportion to their volume/mass. $\endgroup$ – Martin Argerami Jul 26 '15 at 11:40
  • $\begingroup$ The animals obviously don't want to cool down to the surrounding temperature, so they want the most inefficient cooling method possible :-) $\endgroup$ – gnasher729 Jul 26 '15 at 11:54
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    $\begingroup$ And yet all the animals normally thought of as huge, (Elephants, Rhino, and hippos) all live in the warmer climes. So there's more at play here. $\endgroup$ – Dewi Morgan Jul 27 '15 at 9:12

protected by Qmechanic Jul 27 '15 at 0:00

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