Add the milk now or just before drinking?

Question

You pour a cup of very hot black coffee and you have some cold milk from the fridge. You will drink the coffee in ten minutes and you want it to be as hot as possible at that moment.

Should you add the milk immediately, or wait and add it just before drinking? Explain the physics in plain words.

Accepted Answer

Add the milk immediately. Cooling is faster when hotter. A cup loses heat to the room at a rate roughly proportional to the temperature difference between the coffee and the room (Newton's law of cooling). A cup of black coffee at 90 degrees loses heat much faster than a cup of milky coffee at 75 degrees. Compare the two plans. If you wait, the coffee spends ten minutes at its hottest, shedding heat rapidly, and then you cool it further with the milk. If you add the milk first, the coffee immediately drops to a lower temperature and then cools more slowly for the whole ten minutes. In both ca…

Add the milk now or just before drinking?

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