Which of the following is correct for critical temperature ?

Options

A. It is the highest temperature at which liquid and vapour can coexist
B. Boyond this temperature, there is no distinction between the two phases and a gas cannot be liquefied by compression.
C. At this temperature, the gas the liquid phases have different critical densities
D. All are correct

Answer

To solve the question regarding the critical temperature, we need to understand the concept of critical temperature and analyze the given options. Here’s a step-by-step breakdown of the solution:

### Step 1: Define Critical Temperature
Critical temperature (Tc) is defined as the highest temperature at which a substance can exist in the liquid state. Above this temperature, the substance cannot be liquefied, regardless of the pressure applied.

**Hint:** Remember that critical temperature is the maximum temperature for liquid existence.

### Step 2: Analyze the Behavior of Molecules
At temperatures below the critical temperature, the molecules are close enough for intermolecular forces to act effectively, allowing the substance to exist in both liquid and vapor phases. As the temperature increases and approaches the critical temperature, the kinetic energy of the molecules increases, making it harder for these intermolecular forces to hold the molecules together in a liquid state.

**Hint:** Consider how molecular kinetic energy affects phase states.

### Step 3: Identify the Characteristics Above Critical Temperature
Above the critical temperature, the distinction between liquid and gas phases disappears. The substance enters a supercritical state where it exhibits properties of both a liquid and a gas. In this state, the gas cannot be liquefied by pressure alone.

**Hint:** Think about the implications of exceeding critical temperature on phase distinction.

### Step 4: Evaluate the Given Options
1. **Option A:** "It is the highest temperature at which liquid and vapor can coexist." - This is correct.
2. **Option B:** "Beyond this temperature, there is no distinction between two phases and gas cannot be liquefied by compression." - This is also correct.
3. **Option C:** "At this temperature, the gas and liquid phases have different critical densities." - This is incorrect because at the critical point, the densities of the liquid and gas phases become equal.
4. **Option D:** (not provided in the transcript but assumed to be incorrect based on the context).

### Conclusion
The correct options regarding critical temperature are A and B.

**Final Answer:** The correct options are A and B.

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