Question

Two bodies at different temperatures are mixed in a calorimeter. Which of the following quantities remains conserved?

• A. sum of the temperatures of the two bodies
• B. total heat of the two bodies
• C. total internal energy of the two bodies
• D. internal energy of each body

Answer 1

The correct option is C total internal energy of the two bodies

There is no heat lost since the calorimeter is insulated .
If we consider both the liquids together as one system , then the work done by the system is zero.

From first law of thermodynamics,
$\Delta U=Q-W$
$\Delta U=0$

Since there is no change in internal energy, the internal energy of the system remains constant.