This article is from the Puzzles FAQ, by Chris Cole chris@questrel.questrel.com and Matthew Daly mwdaly@pobox.com with numerous contributions by others.

You are just served a hot cup of coffee and want it to be as hot as

possible later. If you like milk in your coffee, should you add it

when you get the cup or just before you drink it?

physics/milk.and.coffee.s

Normalize your temperature scale so that 0 degrees = room temperature.

Assume that the coffee cools at a rate proportional to the difference

in temperature, and that the amount of milk is sufficiently small that

the constant of proportinality is not changed when you add the milk.

An early calculus homework problem is to compute that the temperature

of the coffee decays exponentially with time,

T(t) = exp(-ct) T0, where T0 = temperature at t=0.

Let l = exp(-ct), where t is the duration of the experiment.

Assume that the difference in specific heats of coffee and milk are

negligible, so that if you add milk at temperature M to coffee at

temperature C, you get a mix of temperature aM+bC, where a and b

are constants between 0 and 1, with a+b=1. (Namely, a = the fraction

of final volume that is milk, and b = fraction that is coffee.)

If we let C denote the original coffee temperature and M the milk

temperature, we see that

Add milk later: aM + blC

Add milk now: l(aM+bC) = laM+blC

The difference is d=(1-l)aM. Since l<1 and a>0, we need to worry about

whether M is positive or not.

M>0: Warm milk. So d>0, and adding milk later is better. M=0: Room temp. So d=0, and it doesn't matter. M<0: Cold milk. So d<0, and adding milk now is better.

Of course, if you wanted to be intuitive, the answer is obvious if you

assume the coffee is already at room temperature and the milk is

either scalding hot or subfreezing cold.

Moral of the story: Always think of extreme cases when doing these puzzles.

They are usually the key.

Oh, by the way, if we are allowed to let the milk stand at room

temperature, then let r = the corresponding exponential decay constant

for your milk container.

Add acclimated milk later: arM + blC

We now have lots of cases, depending on whether

r<l: The milk pot is larger than your coffee cup. (E.g, it really is a pot.) r>l: The milk pot is smaller than your coffee cup. (E.g., it's one of those tiny single-serving things.) M>0: The milk is warm. M<0: The milk is cold.

Leaving out the analysis, I compute that you should...

Add warm milk in large pots LATER.

Add warm milk in small pots NOW.

Add cold milk in large pots NOW.

Add cold milk in small pots LATER.

Of course, observe that the above summary holds for the case where the

milk pot is allowed to acclimate; just treat the pot as of infinite

size.

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