People have said that it brings up warmer hot chocolate from the bottom of the mug, but why isn’t the entirety of the hot chocolate the same temperature?
In: Physics
Air currents cool off the surface of the drink. When you stir it, you integrate the cooler drink at the surface with the rest of the cup. Warmer liquid that wasn’t exposed at the bottom of the cup comes up to the surface when stirred and is similarly cooled by the air.
The top of the chocolate cools down from the air, so it’s colder than the bottom. Stirring it brings up the hotter chocolate up from the bottom. Then hot chocolate at the top cools down faster than cold chocolate at the top would.
The mass is cooled by the air via convection at the top. By mixing, you mix the aforementioned mass and cool it further via conduction.
Stirring it also increases how fast heat is lost through the mug as well. Specifically, the transfer of heat from the liquid to the inner surface of the mug.
Another factor is that the spoon you use is (hopefully) made of metal. Metals conduct heat very well, meaning that the spoon itself will quickly absorb some of the liquid’s heat (and thereby become warm or even hot to the touch), and also help transfer heat from the lower layers to the air. Of course, this effect doesn’t require the stirring action itself, it’s enough for the spoon to just sit in the cup.
Heat travels from hotter to cooler objects. If you want hot chocolate to cool down you need to expose it to something that is cooler than it.
The only such portion of liquid hot chocolate in a cup is the one that is at the top, because it is in direct contact with the cooler air. So the top layer cools down fast. The other layers that are underneath can then slowly transfer their heat to the layer at the top, which eventually again lends it to the cooler air. That happens when you don’t stir your chocolate cup at all. Eventually it will cool down on its own because of this heat transfer.
However, you can hasten the cooling by sending the top layer to the bottom and bringing up the bottom layers. This is what stirring does. Now the hotter layers which were earlier buried underneath, come up and in contact with the air and lose heat. As you keep stirring, the hotter layers keep coming up, releasing the heat, in turn, cooling your chocolate pot fast.
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