Every morning, I drink half a liter of coffee with milk. I drink the first cup in the kitchen. But the second cup I like to drink after breakfast in the living room while checking emails. The problem I’m then confronted with is this: the coffee gets cold. But I only like coffee that is at least lukewarm. So how does my coffee stay warm? Let’s do some science…
For this, a thermos or insulated mug would do, but I want to use a normal mug. I’ve got two possibilities to proceed with the milk and the coffee: I could bring the coffee and milk in the living room separately and pour the milk in the coffee at the last moment possible. Or I could pour the milk in the coffee as early as possible and then bring it with me into the living room. Through physics, I know that the timing in adding the milk makes a difference in temperature of the coffee. I like applied science.
Let’s have a look at the principles of cooling. Coffee cools down because of heat loss due to evaporation, air flowing around the cup of coffee, and also heat radiation. The heat loss through evaporation occurs because a lot of energy is needed for liquid to evaporate. This energy is taken from the cup of coffee. The heat loss through the air flowing around the cup of coffee takes place because the cold air around the cup gets heated up, whereby it takes away heat from the coffee. The heat radiation directly emits heat which is lost from the coffee. The greater the difference in temperature between the coffee and the environment, the stronger the effects are.
Therefore, I mix the coffee with milk as early as possible. That causes an immediate drop in the mixture’s temperature. Thus, the difference in temperature between the coffee mixture and the room’s air temperature is smaller. This is why less heat is lost. If I would wait to pour the milk in the coffee, the coffee would be hotter and would lose more heat on the way to the living room. And the temperature drops further when you eventually add the milk.
The same with sugar: put it in as early as possible. Dissolving needs energy, which comes here in form of heat. When the sugar dissolves, the coffee’s temperature drops the same amount, regardless of how hot the coffee is. The earlier the temperature drops due to dissolving, the less total heat is lost.