Attainment of equilibrium can be noticed with the help of constancy of which of the following physical properties ?

Options

A. Intensity of colour
B. Density
C. Pressure
D. All the above

Answer

To solve the question regarding the attainment of equilibrium and the physical properties that indicate this state, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Equilibrium**:
- Equilibrium in a chemical reaction occurs when the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant over time.

2. **Identifying Physical Properties**:
- The question asks which physical property can indicate the attainment of equilibrium. The options provided are:
- A) Intensity of color
- B) Density
- C) Pressure
- D) All of the above

3. **Analyzing Each Option**:
- **Option A: Intensity of Color**:
- In reactions involving colored substances, the intensity of color can change as reactants are converted to products. When equilibrium is reached, the intensity of color remains constant, indicating that the concentrations of reactants and products are stable.

- **Option B: Density**:
- Density can also change during a reaction, but it is not a direct indicator of equilibrium in all cases. It may not remain constant in every reaction scenario.

- **Option C: Pressure**:
- For gaseous reactions, pressure can indicate equilibrium, especially in reactions involving a change in the number of moles of gas. However, it is not universally applicable to all reactions.

- **Option D: All of the above**:
- While all these properties can provide information about the state of a system, only the intensity of color is a direct and consistent indicator of equilibrium in reactions where color change is involved.

4. **Conclusion**:
- Based on the analysis, the most reliable indicator of the attainment of equilibrium among the options provided is the intensity of color. Therefore, the correct answer is **Option A: Intensity of color**.

### Final Answer:
The correct option is **A) Intensity of color**.

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