`H_2O(l) iff H_2O(vap)`
In this equation, the double half arrows indicate that

Options

A. the processes in which the forward direction is more than the backward reaction
B. the processes in which the forward direction is less than the backward reaction
C. the processes in both the directions are going on simultaneously
D. the processes in both the directions are not going on simultaneously

Answer

To analyze the equation \( H_2O(l) \iff H_2O(vap) \), we need to understand the meaning of the double half arrows in this context.

### Step-by-Step Solution:

1. **Understanding the Reaction**: The equation represents the equilibrium between liquid water (\( H_2O(l) \)) and water vapor (\( H_2O(vap) \)). This indicates that water can exist in both liquid and gaseous states.

2. **Significance of Double Half Arrows**: The double half arrows (⇌) signify that the process is reversible. This means that the reaction can proceed in both the forward direction (liquid to vapor) and the backward direction (vapor to liquid).

3. **Equilibrium Concept**: At equilibrium, the rate of the forward reaction (liquid to vapor) is equal to the rate of the backward reaction (vapor to liquid). This means that the concentrations of both phases remain constant over time.

4. **Identifying the Correct Option**:
- **Option 1**: Incorrect, as it suggests that the forward reaction is favored over the backward reaction.
- **Option 2**: Incorrect, as it suggests that the backward reaction is favored over the forward reaction.
- **Option 3**: Correct, as it indicates that both processes (forward and backward) are occurring simultaneously.
- **Option 4**: Incorrect, as it states that the process is not occurring in both directions simultaneously.

5. **Conclusion**: The correct interpretation of the double half arrows in the equation \( H_2O(l) \iff H_2O(vap) \) is that both the forward and backward reactions are happening at the same time, leading to a state of dynamic equilibrium.

### Final Answer:
The double half arrows indicate that the processes in both directions (liquid to vapor and vapor to liquid) are occurring simultaneously.