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A friend of mine, based on casual observation, believes that coffee will cool faster than ordinary hot tap water. Being curious about this I have tried to investigate it myself, but I'm not well versed in physics and thermodnamics. Also, I know that there are many variables to this, so I will attempt to ask the question as specifically as possible:

I have 8oz of water (we can assume that it is impure, but we can not know what those impurities are) that is heated to 200 degrees F in a microwave (let's say we have a really precise microwave that can do that easily) and that is then placed in a standard, throw-away styrofoam coffee cup.

I have an exact duplicate scenario, except that this time we will also brew coffee in the water for several minutes, filter out the coffee using a standard coffee filter (we can assume some coffee particles remain in the water, but we can not know anything about those coffee particles) and then re-heat the water to 200 degrees F before adding it to the foam coffee cup.

If each of these coffee cups are then placed on separate tables in a room-temperature room (let's say 70 degrees F) and allowed to sit, undisturbed, will the liquids cool at roughly equivalent speeds (i.e., will the water and the coffee reach 180 degrees F at roughly the same time) or will one cool faster than the other?

For either scenario, why?

If there's any clarification or wording choices that should be changed, please let me know as I am genuinely curious. My guess is that there's something to do with the specific heats begin different, and that the surface tension of the coffee mixture will be disrupted due to the coffee particles. But, again, I don't really know :-)

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Dissolving things in water does change the specific heat and the conductivity (see e.g. engineeringtoolbox.com/sodium-chloride-water-d_1187.html), so it's possible the cooling rate would change though probably not by very much. –  John Rennie Nov 3 '13 at 16:19
    
You've described a very simple experiment. Why don't you do it? You might like to read about the Mpemba effect. –  rob Jun 5 at 6:55

2 Answers 2

Dissolving the coffe substances, and the particles, cause minor changes in the heat-related properties, but way too little to be noticable.

But there are effects caused by it that may make a difference. The fluid will cool mostly via the top surface, as the cup is well isolated. The interesting effects apply to this surface:

  • There could be foam on the coffee (caused by some of the dissolved substances), which would slow down cooling just like the foam cup does.
  • There could be a partial layer of oil from the coffee - possibly microscopic drops - covering the water surface, and thereby reducing cooling by evaporation.
  • The coffee is of darker color, and will radiate more heat compared to the cup of water in the cup, which has a white bottom.

The effects are of different magnitude regarding the influence on cooling:

  • The effect of foam is certainly stronger per area of surface covered compared to the oil layer, so if there is foam, the coffee will cool down slower, even noticeably slower.

  • If there is no foam, by some percent of the surface covered by oil, I assume the effect of this is stronger than the difference in radiation, so it will still cool down slower - but not noticeably (assuming a small fraction is covered by oil).

  • If there is no oil cover and no foam, the coffee will radiate more heat, and thus cool down faster - but just a little, not noticeably.

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based on casual observation, believes that coffee will cool faster than ordinary hot tap water

Let's revisit the "casual observation" part. Are the initial conditions the same? Water from a kettle will usually be hotter than from a coffee machine. And we tend to do things to coffee that we don't do to water, like add 10% of another liquid (milk) at low temperatures, a dissolvable solid and considerable agitation with a cold metal heat sink. So it is highly likely that the coffee is much cooler than hot water 2 minutes after preparation, but it's nothing to do with the inherent properties of coffee.

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