glass of water

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hi everyone , just wondering - if i had a glass of water at say 0 degrees c and another at 100 degrees c and mixed the two together , would i have a big glass at 50 degrees c ? and does it matter which is mixed to which ?yours thankfully reloaded

[Skids]

Yes, you would.  No, it doesn't.

[paddy_36]

I remember doing this in Physics - it's to do with specific heat capacity, which is defined as the amount of heat required to raise a unit mass of a solid or liquid by one degree Celsius (or Kelvin). So if you have equal amounts of the same substance then you'll get an average temperature when you mix them. I don't think this holds if you mix different substances even if it's equal quantities as they will have different specific heat capacities. Hope this makes sense; is there a physicist in the house?!

[finex]

You called? :-)

You certainly wont get a mixture at 50 degrees. Not unless you make the assumption that the glass container stores no heat energy, which it does.

In order for the water to be at 0 degrees C, the container must be also, or it would affect the temperature of the water. Ignoring the fact that water at 100 degrees C would be boiling away, the glass containing it would again need to be at the same temperature.

So, let's try it each way:

1) You pour the glass of cold water into the glass of boiling water.

You are adding an equal amount of water at 0 degrees and 100 degrees into a container also at 100 degrees. For an instant, the temperature could indeed be 50 degrees. However, the glass is still at 100 degrees, and will begin to heat the water as it cools down. Depending on how thick the glass is, you could easily raise the temperature of the water by several degrees.

(Related note: Ever been in the pub where they have served your pint using a glass straight from the glasswasher? Your pint will be warm and nasty. Just say NO!)

2) You pour the glass of hot water into the glass of cold water.

You are adding an equal amount of water at 100 degrees and 0 degrees into a container also at 0 degrees. For an instant, the temperature could indeed be 50 degrees. However, the glass is still at 0 degrees, and will begin to cool the water as it heats up. Depending on how thick the glass is, you could easily lower the temperature of the water by several degrees.

So, the answer is: It depends!

[landie]

You won't get two glassesful into one glass.

The glass at 0 degrees is solid ice, the glass at 100 degrees is boiling water.

I guess if you had a third container which was at 50 degrees and you were able to simultaneously deposit the contents of the two glasses into it, then you would achieve a 50 degree mixture. This however would lose heat immediately unless artificially maintained.

[finex]

You might get two glasses into one - we don't know how big the glasses are and he didn't say that they were full!

Also pure water at exactly 0 degrees will not be solid ice. It's wierd stuff, and will need to be cooled to just below zero to freeze. As it warms up again, it will remain as ice at zero degrees until warmed slightly more. As I said, wierd stuff...

[thekraut]

Oh, thats a good point. because if the water is ice (which it can be at exactly 0 degrees, not slightly below) you would have to add the heat of melting, which would cause the mixture to end well below 50 degrees.
and as far as i recall from my thermodynamics courses, entropy plays a role as well.

[newtron]

At 1 atm and 0 deg C, both liquid water and ice can coexist.  If you are removing heat from the system, all the water must freeze before the temperature of the system can drop below 0.  If you are adding heat to the system, all the ice must melt before the water temperature will increase above 0.  Also, besides the effect of the temperature of the sides of the container as mentioned by finex, for most homework problems concerning these types of calculations, you must assume that the top, bottom, and sides of the container are perfectly insulated, otherwise it is not a closed system and the outside temperature can affect the temperature of the system. 

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wow : ) thanks to all of yous for your answers , didnt think id get quite such a scientific response : ) just seemed i remembered something years ago of "how " or something that adding the hot to the cold had a slightly different end result from adding the cold to the hot . thanks again , reloaded