MadSci Network: Physics

Subject: Coffee with milk cools more slowly than coffee without milk

Date: Tue Sep 5 11:43:48 2000
Posted by Tommy Johnson
Grade level: grad (non-science) School: UNC-Chapel Hill
City: Chapel Hill State/Province: NC Country: USA
Area of science: Physics
ID: 968168628.Ph

If you have to wait 10 minutes to drink your coffee after pouring it, will it be warmer if you add the milk right away, or if you add it just before drinking it? I experimented with this idea and found that the coffee was on average 2 degrees C warmer if I waited to add the milk. I used identical clear glass coffee cups, 8 oz. of strong coffee, and 1 oz. of whole milk straight from the refrigerator (~12 degrees C); room temperature was around 25 degrees C.

This contradicts the suggested answer given to an earlier inquiry. I'm curious about what might cause these differences in cooling rates. Could it have to do with the color of the liquids and the emissivity as compared to a black body (my physics memory gets pretty vague here)? Or could it do with the specific heat of black coffee vs. coffee with milk? I'm really stretching on this one; any ideas? I tried this experiment three times and came up with the same results each time. I even sampled the coffee and found that I could actually distinguish a 2 degree C temperature difference.

Re: Coffee with milk cools more slowly than coffee without milk

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