|Is it true that hotter coffee will cool off faster?|
|Recently someone told me that they overheard a conversation at work, about whether it's true that the hotter a cup of coffee is, the faster it will cool off.
The answer is:
The hotter an object is compared to it's surroundings, the faster the heat will transfer from the object, to it's surroundings. So a 200 degree cup of coffee will lose it's heat (cool down) faster than a 150 degree cup of coffee will, with both cups sitting on a table in a 72 degree room, for example.
But, the 150 degree cup will still cool to a drinkable temperature, say 125 degrees, well before the 200 degree cup will. Even though the 200 degree cup will lose heat much faster at first, it will always be warmer than the 150 degree cup, until both cups reach room temperature. At that point, both cups will stop losing any more heat, and will remain at room temperature.
If your coffee is always too hot, even after adding milk, keep this in mind: Let the coffee cool before adding the milk. The reason: If you add the milk right away, this will cool it down some, which will slow down the heat loss, making the mixture take longer to cool off to a drinkable temperature.
I actually made a graph of the rate that a cup of coffee cools, but I am not yet equipped to get it onto this page. But here are a few highlights of my actual observations with a thermometer in a 16 oz. ceramic mug of coffee (black) sitting on a table in a 72 degree room:
Starting temperature 200 degrees (F).
Cooling down from 200 degrees to 150 degrees (50 degree drop) took 14 minutes.
Cooling down from 150 degrees to 100 degrees (50 degree drop) took 45 minutes.
Cooling down from 100 degrees to 75 degrees (25 degree drop) took 2 hours.
Cooling down from 75 degrees to 72.4 degrees (2.6 degree drop) took 1 hour.
As you can see, the bigger the temperature difference between the coffee and the room, the faster the coffee cools off. You can also conclude that a 200 degree cup of coffee will take about 14 minutes longer to cool down, than the 150 degree cup!
Above, we were discussing heat loss, or cooling down, of a liquid. There is also a myth about heat gain of liquids. Some think that a cold pot of water will "heat up" or reach a boil faster than a warm pot of water, on a stove burner.
The answer is:
It is true that the cold water will rise in temperature faster at first, once again due to the large difference in temperature, this time between the water and the burner. But as the water warms up, the heat transfer slows down. It will always take longer for colder water reach a given temperature, all other things being equal.
Speaking of boiling water, I've known for a long time that when you heat a pan of water on a stove burner, that the temperature of the water is fairly even, within a few degrees, from the bottom to the top of the pan. And the boiling action first takes place at the point where the heat is being added to the water, which is at the bottom of the pan.
When I heated the water for the coffee experiment, I put the cup of water in a microwave oven, equipped with a turntable. When the water started to boil, I removed it from the oven and did some temperature measurements on it.
To my surprise, the water at the bottom of the cup was only about 160 degrees, while at the top it was over 200 degrees. I had to stir the water, then microwave it more, several times, to get the whole cup of water up to over 200 degrees. This appears to demonstrate that in the oven, the microwave energy which enters near the top of the oven, is drawn to the nearest object, the water near the top of the cup. Only the top layer of water was actually boiling. This seems to explain why water heated for coffee or whatever, in a microwave oven, cools off faster.
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