Q.Which of the following statements is correct?

Concept: Thermodynamic systems are classified by their ability to exchange matter and energy with surroundings.

A thermodynamic system can be:

• Open: exchanges both matter and energy (e.g., an open beaker).
• Closed: exchanges energy but not matter (e.g., a sealed vessel that can conduct heat).
• Isolated: exchanges neither energy nor matter (e.g., an ideal thermos flask).

Now examine each option:

(i) A covered beaker still allows gas exchange and heat transfer — it is open, not truly sealed. Incorrect.

(ii) A closed system by definition exchanges only energy, not matter. This statement claims both are exchanged, which describes an open system. Incorrect.

(iii) Reactants in a sealed copper vessel: matter cannot escape (closed to mass transfer), but copper conducts heat so energy can be exchanged with surroundings. This fits the definition of a closed system. Correct.

(iv) A thermos flask is insulated to prevent heat transfer and sealed to prevent matter transfer — it approximates an isolated system, not merely a closed one. Incorrect.

In thermodynamics, we classify systems by what they exchange with their surroundings. An open system exchanges both matter and energy. A closed system exchanges energy but not matter—the boundary is impermeable to mass flow but allows heat or work. An isolated system exchanges neither.

The key is to recognize what each physical setup actually permits.

1. Examine option (i): covered beaker as an open system

   A covered beaker prevents bulk matter from escaping, so no mass crosses the boundary. However, most covers are not airtight—gases can still leak slowly—but the defining feature of the setup is that it restricts matter flow. More importantly, the statement claims this is an open system, which would require free exchange of matter. A covered beaker is typically treated as closed (or approximately closed), not open. This statement is incorrect.

2. Examine option (ii): closed system exchanges energy and matter

   By definition, a closed system does not exchange matter with the surroundings; it exchanges only energy (as heat or work). The statement says both energy and matter are exchanged, which describes an open system, not a closed one. This is incorrect.

3. Examine option (iii): reactants in a closed copper vessel

   A closed vessel made of copper has rigid, impermeable walls—no matter enters or leaves. Copper is an excellent thermal conductor, so heat can flow freely between the system and surroundings. The system exchanges energy but not matter. This is precisely the definition of a closed system. This statement is correct.

4. Examine option (iv): reactants in a thermos flask (insulated vessel)

   A thermos flask is designed to minimize heat transfer—its walls are insulated. While the vessel is sealed (no matter exchange), the insulation also prevents energy exchange. This setup approximates an isolated system, not merely a closed one. Calling it a closed system is technically loose; the distinguishing feature of a thermos is isolation, not closure. This statement is misleading at best.