Question

A person pours a cup of hot coffee, intending to drink it five minutes later. To keep the coffee as hot as possible, should she put cream in it now or wait until just before she drinks it? Explain.

Short answer

she needs to pour the cream right now because the cream will isolate coffee from the surrounding and helps it to keep the temperature same and the coffee hot.

Step-by-step solution

Given data

A person has poured a cup of hot coffee and intends to drink it five minutes later.

Understanding the transfer of heat  

Heat transfer takes place between two systems if both systems are at different temperatures.

Reason for deciding to add cream now or wait until just before she drinks

The surrounding will act as a sink, and coffee is at a higher temperature, and we know heat transfers from the body which has a higher temperature than to other body which will receive it.

Here coffee will interact with the surroundings and will cool down.

But if we add cream on top of it, the cream will create a barrier between coffee and the surroundings, making it into an isolated system. If we neglect the temperature that will decrease due to the addition of the cream, the process of temperure decease will slow down.

Thus, she needs to pour the cream right now because the cream will isolate coffee from the surrounding and helps it to keep the temperature same and the coffee hot.