Logically, it would seem that adding refrigerator-cold cream to your coffee would result in a cup that is cooler than a cup of black coffee, but the food science geeks at Modernist Cuisine explain why the laws of physics actually prove the opposite: coffee with cream cools about 20% slower than black coffee.
This is true for three reasons. First, black coffee is darker, so it emits heat faster than lighter-colored coffee with cream. The second reason is the Stefan-Boltzmann law, which says that hotter surfaces radiate heat faster. Here's how they explain it:
So let's say you have two cups of coffee that start at the same temperature. You pour cream in cup #1 and the coffee drops in temperature immediately. But the rate at which it loses heat also drops. Meanwhile, the hotter black coffee in cup #2 cools so rapidly that within five minutes the two coffees are at about the same temperature. But you still haven't added the cream to coffee #2! When you do, it cools even more; cup #1 is now the hotter of the two.
The final reason is more viscous liquids — like coffee with cream added — evaporate more slowly, so there is slower heat loss through evaporation.
→ Read more: The Physics of Coffee & Cream at Modernist Cuisine
Interesting, right? Thanks, science!