Question

Three substances are added to a mug to make coffee : Coffee is at ${65}^{o}C$ , Milk at ${65}^{o}C$ and Sugar , which is in thermal equilibrium with coffee. Then ,

• A. Heat will be transferred from sugar to coffee to reach equilibrium
• B. The sugar is in equilibrium with the milk
• C. Heat will be transferred from the coffee to the sugar to reach equilibrium
• D. None of these conclusions are valid

Answer 1

Answer: (B), (C)

The correct options are
B The sugar is in equilibrium with the milk
C Heat will be transferred from the coffee to the sugar to reach equilibrium

Following, the $\text{zeroth law of thermodynamics}$, if a substance is simultaneously in thermal equilibrium with 2 other substances, then the other 2 substances are mutually in thermal equilibrium.

Thus, if Sugar is in equilibrium with coffee and coffee is in equilibrium with milk, then the sugar is in equilibrium with milk. Thus B is correct.

In the heat transfer process, heat may also transfer from coffee to sugar. Thus C is also correct.