Question

The liquid (mercury) meniscus in capillary tube will be convex if the angle of contact is

• A. greater than \(90^{\circ}\)
• B. less than \(90^{\circ}\)
• C. equal to \(90^{\circ}\)
• D. equal to \(0^{\circ}\)

Hint:

Convex meniscus = obtuse angle = non-wetting liquid; Concave meniscus = acute angle = wetting liquid.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The shape of the liquid surface (meniscus) in a capillary tube depends on the adhesive and cohesive forces, which are characterized by the angle of contact.
Step 3: Detailed Explanation:
The angle of contact \(\theta\) is the angle between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid.
1) If \(\theta<90^{\circ}\) (acute), the meniscus is concave (e.g., water in glass). This happens when adhesive forces are stronger than cohesive forces.
2) If \(\theta>90^{\circ}\) (obtuse), the meniscus is convex (e.g., mercury in glass). This happens when cohesive forces are stronger than adhesive forces.
3) If \(\theta = 90^{\circ}\), the meniscus is flat.
Since the question specifies a convex meniscus (mercury), the angle of contact must be obtuse.
Step 4: Final Answer:
The angle of contact is greater than \(90^{\circ}\).