Question

Which of the following is/are true :

• A. A very small amount of mercury on placing over a clean glass plate forms a tiny spherical ball.
• B. A very small amount of water on placing over a clean glass plate forms an oval shaped drop.
• C. Both A & B
• D. None

Answer 1

Answer: (C)

Both A & B

Explanation:

(a) A very small amount of mercury on placing over a clean glass plate forms a tiny spherical ball.
Mercury has a much greater force of cohesion than its adhesive force with glass. Therefore, the molecule of mercury at the edge are pulled inward by the inner molecules. But,the molecules do not move inward as there movement is opposed by the inner molecules. This causes 'surface tension' among the molecules near the edge. Tiny globules are spherical on the account of surface tension because force of gravity is negligible. So, the molecules of mercury near the edge rearrange themselves so that no unbalanced force acting upon the molecules of mercury. In doing so, they take a spherical shape. The bigger globules get flattened from the middle but have round shape near the edges.
(b) A very small amount of water on placing over a clean glass plate forms an oval shaped drop.
The force of attraction between the molecules of water, that is, the force of cohesion is far less than the force of adhesion between the molecules of water and the glass plate and hence it takes an oval shape.