Temperature in tea

Question: Knowing that heat rises and cold falls, why is the hottest part of a cup of tea the bottom and not the top?

Answer:

You have to take into account a number of things.
(a) The density of hot tea is less than cold tea and so it rises to the top
(b) At the top the surface of the tea is in contact with air (usually colder than the tea!)
(c) Heat is lost from the top of the tea to the colder air and so the very top layer cools down
(d) The bigger the surface area the quicker the tea will lose heat from the top
So really the hottest tea is a little below the top layers and since water (tea) is a good insulator it stays hot. Remember that the tea is in a state of continuous motion (convection) – as the very top layer cools it becomes more dense and so falls and therefore is replaced by the layer below it.

I suggest you tray an experiment at school using a set of thermometers at different depths in a tall beaker to test these ideas. Better still see if your physics department has a thermocouple. These devices are much smaller than ordinary thermometers and will give more accurate readings.