Open system (control volume) identification: Which statement correctly defines an open system in thermodynamics?

Introduction / Context:
Thermodynamic analyses distinguish between closed systems (mass fixed) and open systems (control volumes) where mass can flow. Power plants, compressors, turbines, nozzles, pumps, and heat exchangers are typically modeled as open systems during steady operation.

Given Data / Assumptions:

• Open systems allow mass transfer across the control surface.
• Energy can cross as heat and/or work.
• Steady or unsteady conditions may apply, but the definition does not depend on steadiness.

Concept / Approach:
An open system (control volume) exchanges both energy and mass with its surroundings. A closed system allows energy transfer as heat or work but not mass transfer. An isolated system allows neither energy nor mass transfer. Hence the only option that matches the definition of an open system is the one stating that heat, work, and mass may all cross the boundary.

Step-by-Step Solution:

Verification / Alternative check:
Examples: air compressor (mass in and out, shaft work in, heat to surroundings), gas turbine (mass in/out, heat from combustion, shaft work out) all fit the open-system definition.

Why Other Options Are Wrong:

• A: Describes a closed system.
• B: Mass crosses but claims no heat/work—too restrictive and not the defining criterion.
• D: Describes an isolated system.

Common Pitfalls:
Equating open system with steady flow only; they are related but not synonymous.

Final Answer:
both the heat and work as well as mass of the working substance crosses the boundary of the system