If a pressure greater than atmospheric pressure is applied to pure water and a solution, its `Psi_w` value

Options

A. increases
B. decreases
C. remains constant.
D. will be equal to `Psi_p`

Answer

To solve the question regarding the effect of applying pressure greater than atmospheric pressure to pure water and a solution on their water potential (Ψw), we can break it down step by step.

### Step-by-Step Solution:

1. **Understanding Water Potential (Ψw)**:
- Water potential (Ψw) is defined as the potential energy of water in a system compared to pure water. It is influenced by two components: solute potential (Ψs) and pressure potential (Ψp).
- The formula for water potential is:
\[ Ψw = Ψs + Ψp \]

2. **Analyzing Pure Water**:
- For pure water, the solute potential (Ψs) is zero because there are no solutes present.
- At atmospheric pressure, the pressure potential (Ψp) is also zero.
- Therefore, the water potential of pure water at atmospheric pressure is:
\[ Ψw = 0 + 0 = 0 \]

3. **Effect of Applying Pressure**:
- When pressure greater than atmospheric pressure is applied to pure water, the pressure potential (Ψp) becomes positive.
- Since Ψs remains zero (as there are still no solutes), the water potential (Ψw) will now be:
\[ Ψw = 0 + Ψp \quad (where \ Ψp > 0) \]
- This indicates that Ψw increases as Ψp increases.

4. **Analyzing a Solution**:
- For a solution, the solute potential (Ψs) is negative (since it contains solutes).
- However, when pressure greater than atmospheric pressure is applied, the pressure potential (Ψp) is still positive.
- The water potential for the solution can be expressed as:
\[ Ψw = Ψs + Ψp \]
- The exact change in Ψw will depend on the values of Ψs and Ψp, but generally, applying pressure will increase Ψw.

5. **Conclusion**:
- In both cases (pure water and a solution), applying pressure greater than atmospheric pressure results in an increase in water potential (Ψw).
- Therefore, the correct answer to the question is that the Ψw value **increases** when pressure greater than atmospheric pressure is applied.

### Final Answer:
The water potential (Ψw) value **increases** when pressure greater than atmospheric pressure is applied to pure water and a solution.