Question

I have a glass of water at room temperature, kept inside a closed room. I get a hot mug of coffee, keep it on another table, and get so busy I completely forget about it. As the coffee cools down, what happens to the temperature of the glass of water?

• A. Stays absolutely unaffected, since it is on another table
• B. The coffee cools down; the water also cools down with time
• C. The water gets warmer
• D. Cannot say from the given information

Answer 1

The correct option is C

The water gets warmer

Let us assume the room is perfectly closed and no heat is being gained from or lost to the environment outside.

Here we have not two but three bodies, thermally interacting inside the room - the glass of water (initially at room temperature of 23⁰C), the air in the room, and the hot mug of coffee.

At the coffee cools down, it slightly increases the hotness of the air in the room. Thus, the room temperature itself increases after some time (say, to ~24⁰C); this means the glass of water is now cooler (~1⁰C) than the air around it. Due to this temperature difference, the water will get slightly warmer until it has the same new temperature as the air.