Assertion : When ice and water are kept in a perfectly insulated thermos flask at 273 K and the atmospheric pressure, there is no change in mass of ice and water.
Reason : The system is in static equilibrium.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that when ice and water are kept in a perfectly insulated thermos flask at 273 K (0°C) and atmospheric pressure, there is no change in the mass of ice and water.

- **Explanation**: In a perfectly insulated system, there is no heat exchange with the surroundings, meaning that the temperature remains constant. At 273 K, ice and water can coexist in equilibrium. Since there is no heat transfer, the mass of ice and water remains constant over time.

### Step 2: Understand the Reason
The reason given is that the system is in static equilibrium.

- **Explanation**: Static equilibrium implies that there are no changes occurring in the system. However, in the case of ice and water at 0°C, there is a dynamic equilibrium where ice melts into water and water freezes into ice at the same rate. This means that while the overall mass of ice and water remains constant, there is still a continuous exchange between the two states (ice and water). Therefore, the system is not in static equilibrium; it is in dynamic equilibrium.

### Step 3: Conclusion
- The assertion is **true** because the mass of ice and water does not change in a perfectly insulated system at 273 K.
- The reason is **false** because the system is not in static equilibrium; it is in dynamic equilibrium.

### Final Answer
The correct option is C: The assertion is true, but the reason is false.

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