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Angle of Contact

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Estimated time: 5 minutes
CBSE: Class 12
Maharashtra State Board: Class 11

Definition: Angle of Contact

The angle between the surface of the solid and the tangent drawn to the surface of the liquid at the point of contact on the side of liquid is called the angle of contact of that liquid with that solid.

OR

The angle enclosed between the tangents to the liquid surface and the solid surface inside the liquid, both the tangents being drawn at the point of contact of the liquid with the solid, is called the angle of contact.

Maharashtra State Board: Class 11

Formula: Angle of Contact Conditions

Case 1: θ < 90° (Concave Meniscus)

Fa > \[\frac {F_c}{\sqrt 2}\]
 
Adhesive force > Cohesive force → Liquid wets solid → Level rises in capillary

Case 2: θ = 90° (Plane/Horizontal Meniscus)

Fa = \[\frac {F_c}{\sqrt 2}\]
Adhesive force = Cohesive force → Liquid does not wet solid → Level remains unchanged
 
Case 3: θ > 90° (Convex Meniscus)
Fa < \[\frac {F_c}{\sqrt 2}\]
Adhesive force < Cohesive force → Liquid does not wet solid → Level falls in capillary
CBSE: Class 12

Definition: Concave Meniscus

When adhesive forces are stronger than cohesive forces, the curved liquid surface formed when the liquid is in contact with a solid is called a concave meniscus.

CBSE: Class 12

Definition: Convex Meniscus

When cohesive forces are stronger than adhesive forces (e.g., mercury in glass), the curved liquid surface formed where cos θ is negative and the liquid level is lower is called a convex meniscus.

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