Question

The angle of contact is the angle between ________.

• A. the normals to the liquid surface and the container wall
• B. the liquid surface and the container wall
• C. the tangent to the liquid surface and solid surface within the liquid at the point of contact
• D. the liquid surface and solid surface outside the liquid
• E. the line joining the centres of curvature of the liquid meniscus

Hint:

Angle of contact is acute for wetting liquids (like water on glass) and obtuse for non-wetting liquids (like mercury).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept
The angle of contact is a measure of the wetting ability of a liquid on a solid surface. It is the angle formed by the liquid at the three-phase boundary where the liquid, gas (or vapor), and solid intersect.
Step 2: Detailed Explanation
Let's analyze the precise definition of the angle of contact.
Consider a drop of liquid on a solid surface, or liquid in a container. At the point where the liquid surface meets the solid wall, we can draw two tangents:
1. A tangent to the liquid surface at the point of contact.
2. The solid surface itself acts as a tangent to the solid at that point.
The angle of contact (\(\theta\)) is defined as the angle between these two tangents, measured \textit{inside the liquid}.
Now let's evaluate the options based on this definition:
(A) the normals to the liquid surface and the container wall: Incorrect. It is defined by tangents, not normals.
(B) the liquid surface and the container wall: This is too vague. The liquid surface is curved, so we must use a tangent to define the angle.
(C) the tangent to the liquid surface and solid surface within the liquid at the point of contact: This is the precise and correct definition. The phrase "within the liquid" is crucial as it specifies which of the two possible angles is the correct one.
(D) the liquid surface and solid surface outside the liquid: Incorrect. The angle is measured inside the liquid.
(E) the line joining the centres of curvature of the liquid meniscus: This is unrelated to the angle of contact.
Step 4: Final Answer
The angle of contact is the angle between the tangent to the liquid surface and the solid surface, measured inside the liquid at the point of contact.