QUESTION #418

# 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 person!

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 be same.

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 two!

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!
Answered by: 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.