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Q & A: milk in your tea

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Most recent answer: 10/22/2007
Q:
If I add milk to my coffee or tea will the milk rise the temperure of the cup of tea, coffee or plain boiling water.
- Chris Sexton (age 54)
Mortdale NSW Australia
A:
Chris-

I'm assuming that your milk either is at room temperature or somewhat cooler. Since it's much cooler than the hot tea, coffee, etc, it will LOWER their temperature. (There aren't any chemical reactions between the milk and the water that would lead to significant heating, if any.)

More precisely, the temperature of the combined milk and tea will be a weighted average of their initial temperatures. Milk and tea/coffee have almost the same specific heats, and so we can express the final temperature as

T(final) = (T(milk)*m(milk)+T(tea)*m(tea))/(m(milk)+m(tea))

where T is the temperature, and m(milk) is the mass of the milk and m(tea) is the mass of the tea. You can use the volumes of the milk and tea instead of the masses as an approximation. The milk will warm up to this final temperature, and the coffee/tea will cool down to this final temperature, when both are in equilibrium.


Perhaps what you're thinking of is another effect. Water with milk (or any water solution with some salt, sugar, etc.) can be heated to a higher temperature before it boils than can pure water. So if you reheat your drink in a microwave, you can get it a little hotter before it boils away if it has some milk in it.


Mike W.

(published on 10/22/2007)

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