Cup of Coffee
Science brain teasers require understanding of the physical or biological world and the laws that govern it.
You are served a hot cup of coffee and roomtemperature cream at a restaurant. You want to wait a few minutes before you drink the coffee, and you want it to be as hot as possible when you drink it. Should you pour the cream in the coffee:
a) Immediately
b) Just before you drink it
c) It doesn't matter
( "don't add any cream" is not an option)
Answer
The driving force for heat transfer is temperature difference. The coffee by itself is very hot and will therefore cool down at a fast rate. Then once the cream is added the temperature will drop even more. If the cream is added immediately, then the temperature will drop initially but will then drop at a slower rate since the coffee with cream is cooler than the coffee alone and therefore the driving force for heat transfer is less.
As an added bonus, adding the cream will increase the mass of the contents of the cup. A larger mass takes longer to cool down than a smaller one at the same temperature.
Therefore, the cream should be added immediately.
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Comments
cathalmccabe
Jun 18, 2002
 I'm not sure about where you say "If the cream is added immediately, then the temperature will drop initially but will then drop at a slower rate since the coffee with cream is cooler than the coffee alone and therefore the driving force for heat transfer is less."
There might be less driving force, but the coffee and cream is already at a much lower temperature. Would it not be true that when the coffe on it's only gets to the same temperature as the coffe and cream drops to initially that its rate of change would be less too, because it is at a lower temp, but during this time the coffee and cream has dropped even further. I'm wonderinging what would happen depending on the volumes involved, for example 1 litre coffee, 1ml of cream, or how hot the coffee is, although the size of a cup of coffee and cream added are obviously more specific!
I agree with your answer, just not fully with your reasoning! I think its a brilliant teaser by the way! Good work! 
bobbrt
Jun 18, 2002
 I tried to come up with a better explanation for this, but I'm not having much luck. I'm afraid that any nonbelievers will just have to go on faith (unless someone else can explain it better). By the way Cathal, thanks for the compliment but this teaser is far from being an original. You'll have to thank the good people at  oh, wait  I get what's going on here! You're trying to get me to reveal my sources!! Well you can just wait until my pet pig "Muffin" flies to China under his own power!! HA HA HA...I'll NEVER TELL! NEVER!!!!!!! 
Dazza
Jun 18, 2002
 I saw this problem on a kids science show about 20 years ago. From memory, I think the presenter said that there would be no difference, but I could be wrong. However, he did say that if the cream was high in fat it would leave a thin film of insulating oil in the top of the coffee. Hence the coffee would be slightly warmer if you added the cream first. 
WizardMagus
Jun 19, 2002
 Heat loss is an exponential decay graph. The hotter the object is, the faster it cools. As it cools more and more, it starts to cool less rapidly. When the cream is added immediately, it brings the temperature down to a lower level, meaning the heat will be lost less rapidly. If you wait for the coffee to cool first, and then add the cream, the temperature change of the coffee would be less because the two substances are already closer together in temperature scale. Therefore, it shouldn't really matter, because one way you lose heat in the mixture, one way you lose heat into the air. The ammount of heat lost in each option should be equal, if you wait the same ammount of time with each trial. 
Mogmatt16
Jun 19, 2002
 who would really do all that thinking about when to put in their cream? it doesn't seem like much of a diffrence, but I may be wrong... 
cathalmccabe
Jun 21, 2002
 It was the great thinkers of ancient times who developed simple games for the most complicated problems they came across as a method to solve them. John Nashes @Game Theory@ is a modern day example (film: A beautiful mind based on him) Its little trivial things like this that can solve much more complicated issues, and its a lot easier to experiment on a cup of coffe for temperature than say a Power Plant, or whatever 
dewtell
Aug 08, 2002
 Actually, I think the "driving force" aspect of the answer
is wrong. Suppose that the coffee loses half the temperature difference
between itself and the room every minute (a nice exponential decay curve),
and that it starts out with a 128degree difference (coffee at 198 degrees, room at 70).
The temperature of the cream isn't going to be changing, so suppose that it reduces the
coffee temperature difference by 1/4th (there is 3 times as much coffee as cream).
Then if you let the coffee cool first, after 4 minutes the difference is 1/16th what it
started with, so it's an 8 degree difference (78 degrees). Adding the cream now reduces
the difference by 1/4, to 6 degrees. But if you added the cream first, you would have dropped
the temperature difference to 96 degrees difference. Letting this cool for 4 minutes likewise
drops the difference to 1/16th  to the same six degree difference. The temperature loss by cooling
is less, but the final temperature is the same.
However, the "added bonus" part probably still applies  the surface area through which the heat is
lost (primarily the top) remains the same, but the volumn holding the heat has increased. So you
probably do get some effect due to this.

angel11
Oct 03, 2002
 I got it right, but I didn't even know why that was the answer. You all are geniuses. 
jimbo
Nov 08, 2003
 The answer supplied is quite correct. The puzzle as I heard originally came from a series called "Why is it so?" by Professor Julius Sumner Miller. (I think he is a physicist). It can be modelled with differential equations but it is probably not necessary if you consider the answer given that heat loss is greatest when temperature difference is greatest. Therefore the sooner the coffee is brought closer to room temperature, the slower will be the heat loss. 
curtiss82
Dec 25, 2003
 Dewtell, your math is totally wrong. I will show you the correct math. Start off with an overall energy balance and simplify.
In  Out + Source Sink = Accumulation.
=> Out = Accumulation
In this case, I will assume that the Biot number is less than .1 and thus I won't have to worry about any temperature gradients in the coffee. To also simplify the problem, I will assume that heat will only be convected away from the cup at the liquid surface and I will assume that the surface area doesn't change when the cream is added.
Going back to the equation, after adding in Newton's Law of Cooling and the accumulation term, it becomes:
A*h*(TTc) = V*ro*Cp*dT/dt
A = Area of heat transfer
h= heat transfer coefficient
T= Temperature of coffee
Tc = Air temperature
V= Volume of Coffee
ro= Density
Cp= Heat capacity
dT/dt= Rate of change of Temperature with respect to time
This differential equation can be separated and integrated resulting in an equation that gives the temperature versus time:
T = Tc + (ThTc)*exp((A*h*t)/(V*ro*Cp))
Th= Initial temp. of coffee
I will assume that all the physical properties of coffee are the same as water and I will make a logical guess as to the size of the cup, the initial temperature, and the magnitude of the heat transfer coefficient.
I will also assume ideal mixing so I can calculate the temperature of the coffee after mixing.
No matter what time I plug in I get that the coffee that had the cream added immediately was slightly warmer than the coffee that had the cream added right before drinking. 
joanie
Dec 26, 2003
 Hey am I the only one who knew the answer just from years of actually drinking coffee with cream? 
vikingboy
Sep 02, 2005
 While no one may ever read this as I'm responding two years later than the last response, there is one fundamental issue that many of you are missing.
Stirring a liquid is one of the fastest ways to cool it down. By stirring at the start you lose a lot of heat.
Add the cream, but don't stir. 
beegum
Dec 13, 2005
 Bassically, since the coffee will need to use its energy on the cream anyway, add the cream immediatelly... 
JasonD
Aug 12, 2008
 Here's an explanation that's not as precise as dewtell's, but it might be helpful.
 In terms of mass and chemical composition, the coffee + cream mixture is identical regardless of how it's done.
 The total energy of the system starts out the same in either scenario (obviously).
 The cream, at room temperature, neither gains nor loses thermal energy.
Therefore: We need only consider the net energy loss from whatever is in the coffee cup.
The hotter coffee (waiting to add the cream) loses energy at a faster rate, because it has a greater temperature difference compared to the surrounding air.
The imprecision in this scenario comes from ignoring the increased convection area of the coffee + cream system. I.e., increasing the volume raises the fluid level, thus increasing the surface area of the cylinder formed by the liquid.
This is likely to be neglible because of the insulating effects of coffee containers  most all of the energy is being lost from the discshaped top of the cylinder, and that disc does not increase in size. 
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