Explain why (a) The angle of contact of mercury with glass is obtuse, while that of water with glass is acute. (b) Water on a clean glass surface tends to spread out while mercury on the same surface tends to form drops. (Put differently, water wets glass while mercury does not.) (c) Surface tension of a liquid is independent of the area of the surface (d) Water with detergent disolved in it should have small angles of contact. (e) A drop of liquid under no external forces is always spherical in shape
a)
The adhesive force between glass and water molecules is greater as compared to the cohesive force between water molecules. This makes a concave meniscus for the surface of water. Thus, water makes an acute angle from the glass surface.
The adhesive force between glass and mercury molecules is less as compared to the cohesive force between mercury molecules. This makes a convex meniscus for the surface of mercury. Thus, mercury makes an obtuse angle from the glass surface.
b)
Water molecules make acute angles with glass. They have a weak force of attraction between themselves and a strong force of attraction toward solids, thus they spread out.
In the case of mercury molecules which make an obtuse angle with glass have a strong force of attraction between themselves and a weak force of attraction toward solids. Hence, they form drop.
c)
Surface tension is defined as the force acting per unit length at the interface between the plane of a liquid and any other surface. It depends upon the nature of liquid not on the area of contact.
d)
Water with detergent dissolved in it has small angles of contact θ . For a small θ , there is a fast capillary rise of the detergent in the cloth. The capillary rise of a liquid is directly proportional to the cosine of the angle of contact. If θ is small, then cos θ will be large and the rise of the detergent water in the cloth will be fast.
e)
A liquid tends to acquire the minimum surface area. Since, the surface area of a sphere is minimum for a given volume therefore, under no external forces, liquid drops always take spherical shape.
a)
The adhesive force between glass and water molecules is greater as compared to the cohesive force between water molecules. This makes a concave meniscus for the surface of water. Thus, water makes an acute angle from the glass surface.
The adhesive force between glass and mercury molecules is less as compared to the cohesive force between mercury molecules. This makes a convex meniscus for the surface of mercury. Thus, mercury makes an obtuse angle from the glass surface.
b)
Water molecules make acute angles with glass. They have a weak force of attraction between themselves and a strong force of attraction toward solids, thus they spread out.
In the case of mercury molecules which make an obtuse angle with glass have a strong force of attraction between themselves and a weak force of attraction toward solids. Hence, they form drop.
c)
Surface tension is defined as the force acting per unit length at the interface between the plane of a liquid and any other surface. It depends upon the nature of liquid not on the area of contact.
d)
Water with detergent dissolved in it has small angles of contact
e)
A liquid tends to acquire the minimum surface area. Since, the surface area of a sphere is minimum for a given volume therefore, under no external forces, liquid drops always take spherical shape.
