When to add the milk

11 November 2007


Measuring the temperature both with a normal thermometer and one connected to a computer.



TeaSome TeaThermometerA thermometer which measures from 30-100°C
MilkSome milkA measure

A measure of some sort, that will measure the milk

Mugs2 identical mugs


We are trying to compare what happens to the temperature of milky tea when you add the milk at the beginning or at the end of a wait.

So make 2 identical cups of tea and pour a measured amount of milk into the first at the beginning of the experiment.

Wait 15 minutes then add the milk to the second cup of tea and measure the temperature of both cups.


You should find that once you add the milk to the second cup, the cup which you added the milk to first is the warmest.

Below you can see a graph of the temperature of the tea against time made with a mutilated electronic thermometer plugged into a computer. You can see that the tea which has milk added first cools down more slowly than the tea with no milk in it. So when you add the milk to the second cup of tea it ends up colder.

Tea temperature


Adding the milk lowers the temperature of the tea but it doesn't do this by removing energy, it does it by spreading the energy around.

The hotter the tea the bigger the difference in temperature is between it and the room. This big temperature difference means that heat energy is lost more quickly so the tea without milk in cools more quickly than the the one you added milk to at the start. So during the experiment the cup with no milk will loose more heat, but should still always be warmer than the cup with milk. That is until you add milk to it when it will become significantly colder.

Hot TeaWarm tea
Hot tea is much warmer than the surroundings so it looses heat quickly.Warm tea is only slightly warmer than its surroundings so it only looses heat slowly.

Because in the end both cups of tea are identical, at the end the one with the most energy will be the hottest. This means that the tea which had the milk added first has lost less energy so will end up warmer.


you needed a fourth cup with tea and milk that was heated to the same temperature as the tea that didn't ever have milk in it, so we could see four lines on the chart

You didn't take the warming of the milk into account. If you take it out of the fridge just before adding it in both cases, then the answer is indeed trivial.


Thanks. My wife and I were debating this and Google and you helped out!

Great scientific method and explanation!
Was wandering about this today and you did the test perfectly.
Undeniable results! Good science!

My husband and I were debating this too! You seem like a smart intellectual, want to hang out some time? I can see us becoming good buddies.

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