When to add the milk

We were asked whether if you are going to leave your tea for a while should you add the milk at the start or end of your wait? Which way gives the hottest tea?

What you need

What to Do

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.

What may Happen

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.

What is going on?

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.

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.