Which of the following is correct ?

Options

A. If angle of contact is obtuse, the liquid rises in capillary
B. If angle of contact is acute , then liquid wets solid .
C. If angle of contact is obtuse , the shape of liquid menisus is convex upwards .
D. Surface tension of liquids decreases with rise in temperature .

Answer

To solve the question regarding the correctness of the statements related to capillarity and surface tension, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: If the angle of contact is obtuse, the liquid rises in capillary.**
- The angle of contact (θ) is defined as the angle between the tangent to the liquid surface and the solid surface inside the liquid. When θ is obtuse (greater than 90 degrees), cos(θ) becomes negative.
- The height of the liquid column (h) in a capillary tube is given by the formula:
\[ h = \frac{2T \cos \theta}{R \rho g} \]
- Since cos(θ) is negative for obtuse angles, h will also be negative, indicating that the liquid will not rise but rather fall inside the capillary tube.
- **Conclusion:** This statement is **incorrect**.

2. **Statement 2: If the angle of contact is acute, then the liquid wets the solid.**
- For an acute angle (θ < 90 degrees), cos(θ) is positive. This results in a positive height (h) in the capillary tube, indicating that the liquid rises.
- The formation of a concave meniscus occurs, which indicates that the adhesive forces between the liquid and solid are greater than the cohesive forces within the liquid.
- **Conclusion:** This statement is **correct**.

3. **Statement 3: If the angle of contact is obtuse, then the shape of the liquid meniscus is convex upwards.**
- As previously discussed, when θ is obtuse, the meniscus formed will indeed be convex upwards. This is because the liquid does not wet the solid surface, leading to a convex shape.
- **Conclusion:** This statement is **correct**.

4. **Statement 4: The surface tension of liquids decreases with the rise in temperature.**
- Surface tension is related to the intermolecular forces within the liquid. As temperature increases, these intermolecular forces weaken, leading to a decrease in surface tension.
- **Conclusion:** This statement is **correct**.

### Final Conclusion:
- The correct statements are **2, 3, and 4**. Statement 1 is incorrect.