I noticed that pouring hot coffee into cold cream results in a higher final beverage
temperature than pouring the cold cream into the hot coffee. Can you explain the physical
principle behind this observation?
This is a wonderful question! It shows that you are a very clever and observant
It would seem that it should not matter whether you poured the cream into the
coffee or the coffee into the cream. The final temperature in either case should
However, there are some important things to be considered. First, are the amounts
of coffee and cream the same? If you pour exactly the same amounts of each into
the other than the final temperature should be te same for both. For example, if I
pour 75mL of cream into 300mL of coffee or if I pour 300mL of coffee into 75mL of
cream the result should be a mixture of the same temperature in both cases.
But, as your question implies, this is not the case! What could be causing the
difference? Let's look at this again. In the first case I am pouring 75mL of cream
into hot coffee. The hot coffee is in a cup which is also hot because of the
coffee. Hmm. . . the cream will cool this somewhat but there is a awful lot of
heat in the cup itself!
In the second case I am pouring hot coffee into a cup that already has 75mL of
cold cream. Now the cup itself is cooler. The heat from the coffee will have to
heat up both the cream and the cup. This mixture should be the cooler one of the
But you question says it is the other way around! I suspect that if you waited a
little longer to measure the temperature of the second mixture you would find that
it is the cooler of the two. At least that was the case for me when I did this
experiment.. The initial temperature was higher as your question implies; however,
in my case, when I waited for the cup to get hot too the temperature of the second
mixture was cooler. After a short time though, both were the same temperature!
As I come to the end of this answer it occurs to me that I should also test this in
styrofoam cups. Being at the house I used identical ceramic cups. Why don't you
try this with styrofoam? Let us know what you discover!
Tom Young, B.A., Science Teacher, Whitehouse High School
This observation does not make sense with current ideas of thermodynamics, before
you can make a statement like that you need to perform the experiment again but
this time stir the coffee and allow it to reach thermal equilibrium. Do this for
both situations and you will find that it doesn't make a difference, both
situations will be at the same temperature as long as the starting temperatures
(coffee and cream) remain the same for both observations. And you measure the
temperature at the same time after the start of the experiment for each.
Your observation would be correct if the original temperatures weren't the same for
both experiments (coffee and cream) or you didn't allow the coffee to reach thermal
equilibrium. If you want to do the experiment again you must make sure that you do
exactly the same for both i.e. same original temperature, same volumes of liquid,
everything must be the same for it to be fair.
Pouring cold cream into hot coffee would give the impression that the coffee is
cooler because the cold liquid is on top, where you drink. And visa versa for
coffee in to cream. This is because thermal equilibrium has not been reached.
Dan Summons, Physics Undergrad Student, UOS, Souhampton
'Restlessness and discontent are the first necessities of progress.'