ANS:C - flow rates & composition both are time dependent.

A chemical process is said to occur under unsteady state if the flow rates and composition both are time-dependent. In a steady-state process, the system's properties do not change with time, meaning the flow rates and compositions remain constant. However, in an unsteady state, these parameters fluctuate over time, indicating dynamic changes within the system. Therefore, the correct option is: flow rates & composition both are time dependent. When we say that flow rates and composition are both time-dependent in an unsteady state process, it means that the amount of material flowing into and out of the system, as well as the composition of that material, change over time. Here's a more detailed explanation:

1. Flow Rates (or Mass Flow Rates):
   • Flow rates refer to the amount of material (mass, volume, or moles) that passes through a certain point per unit time. In an unsteady state process, these flow rates are not constant over time. They may increase or decrease as the process progresses.
   • For example, if you're considering a chemical reactor, the flow rate of reactants into the reactor might vary over time due to changes in reaction rates, feed rates, or operating conditions.
2. Composition:
   • Composition refers to the relative amounts of different components or species present in the material flowing through the system. In an unsteady state process, the composition of the incoming and outgoing streams can change with time.
   • This change in composition can occur due to various factors such as changes in feed composition, reaction rates, temperature, pressure, or mixing within the system.

In summary, when both flow rates and composition are time-dependent, it indicates that the process is not in a steady state and is undergoing dynamic changes over time. This could be due to various factors such as changes in operating conditions, external disturbances, or transient behavior within the system. Analyzing and understanding these time-dependent variations is essential for optimizing and controlling the process effectively.