If a system changes from a state P1V1 to P2V2 by two paths then the quantity which remains unchanged is _.

Explanation

The quantity that remains unchanged is known as a state function, which depends only on the initial and final states of the system and is independent of the path taken between those states. The only state function among the given options is dQ - dW.dQ is the heat added to the system, and dW is the work done on the system. The first law of thermodynamics states that the change in internal energy of a system is equal to the heat added to the system minus the work done on the system, i.e., ΔU = dQ - dW. Thus, the quantity dQ - dW is equal to the change in internal energy, which is a state function.