If the vapour pressure of solutions of two liquids are less than those expected ideal solution they are said to have:

Options

A. negative deviation from ideal behaviour
B. positive deviations from ideal behaviour
C. ideal behaviour
D. positive deviation for lower concentration and negative deviations for higher concentration

Answer

To solve the question, we need to understand the concept of vapor pressure in solutions and how it relates to ideal and non-ideal behavior.

### Step-by-Step Solution:

1. **Understanding Vapor Pressure**:
- When two liquids are mixed, the vapor pressure of the resulting solution can be affected by the interactions between the molecules of the different liquids.

2. **Ideal Solutions**:
- An ideal solution is one where the vapor pressure of the solution is directly proportional to the mole fraction of the components, following Raoult's law. In an ideal solution, the interactions between the different molecules (A and B) are similar to the interactions in the pure components (AA and BB).

3. **Non-Ideal Solutions**:
- In non-ideal solutions, the interactions between the different types of molecules (AB) can be different from the interactions in the pure components. This can lead to either positive or negative deviations from Raoult's law.

4. **Negative Deviation**:
- If the vapor pressure of the solution is less than what is expected from an ideal solution, this indicates that the AB interactions are stronger than the AA and BB interactions. This results in a lower vapor pressure than predicted, which is known as a negative deviation from Raoult's law.

5. **Conclusion**:
- Therefore, if the vapor pressure of solutions of two liquids is less than that expected from an ideal solution, they are said to exhibit **negative deviation from ideal behavior**.

### Final Answer:
The correct answer is **negative deviation from ideal behavior**.

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