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
Explanations
(a) The angle of contact of mercury with glass is obtuse, while that of water with glass is acute
The angle of contact depends on the relative strength of cohesive forces (within the liquid) and adhesive forces (between the liquid and solid). Mercury has strong cohesive forces due to metallic bonding, which dominate over the adhesive forces with glass, leading to an obtuse angle. Water, with strong adhesive forces to glass, forms an acute angle.
(b) Water on a clean glass surface tends to spread out while mercury on the same surface tends to form drops
This is due to the wetting properties of the liquids. Water has high adhesive forces with glass, allowing it to spread out and wet the surface. Mercury, with stronger cohesive forces than adhesive forces, minimizes its surface area by forming droplets, thus avoiding wetting the glass.
(c) Surface tension of a liquid is independent of the area of the surface
Surface tension arises from molecular interactions at the interface between a liquid and another medium (e.g., air). It depends on the liquid's intermolecular forces but not on the size or area of the surface. The force per unit length remains constant, regardless of the area.
(d) Water with detergent dissolved in it should have small angles of contact
Detergents reduce the surface tension of water by weakening cohesive forces among water molecules. This enhances adhesive forces between water and other surfaces, resulting in a smaller contact angle and better wetting of the surface.
(e) A drop of liquid under no external forces is always spherical in shape
A sphere minimizes the surface area for a given volume, which minimizes the surface energy due to surface tension. In the absence of external forces like gravity, this shape is the most stable configuration for a liquid drop.
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