Q9.2: NCERT Physics - Part 2 MECHANICAL PROPERTIES OF FLUIDS 9.2 Solution: Explain why (a) The angle of contact of mercury with glass is…

**Answer:** (a) Cohesive vs adhesive forces. (b) Same reason. (c) Surface tension depends on interface length, not area. (d) Detergent reduces surface tension of water. (e) Sphere has minimum surface area for a given volume. **Step-by-Step Explanation:** (a) For mercury-glass, the cohesive forces (mercury-mercury) are stronger than adhesive forces (mercury-glass), so the liquid does not wet the glass, giving an obtuse angle. For water-glass, adhesive forces are stronger, giving an acute angle [citation:31]. (b) Water wets glass because adhesive forces are stronger, so it spreads. Mercury does not wet glass because cohesive forces dominate, so it forms drops [citation:31]. (c) Surface tension is the force per unit length along the interface between a liquid and another surface. It depends on the nature of the interface, not on the area of the surface [citation:31]. (d) Detergents are surfactants that reduce the surface tension of water. A lower surface tension means a smaller angle of contact, allowing water to spread and wet surfaces more effectively [citation:31]. (e) A liquid drop tends to minimize its surface energy. For a given volume, a sphere has the minimum surface area. Hence, in the absence of external forces, a drop becomes spherical [citation:31].