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)
