Explain these observations: (a) water forms globules on a greasy plate but not on a clean one; (b) small bubbles on the surface of water cluster together.

(a) When water is present on a greasy plate, the adhesive forces that exist between the water molecules and grease molecules are less than the cohesive force exerted between the water molecules. Due to the strong cohesive forces, the water present in the grease plate is tending to form globules. Therefore, water forms globules present on a grease plate.On the other hand, the presence of water on a clean plate makes the adhesive force between the water molecules present in the glass is greater than the cohesive force. So, globules are not formed on the clean plate.
(b) When small bubbles are present on the surface of water, the adhesive force between the water molecules and the air molecules present on the bubbles is greater than the cohesive force between the water molecules. Thus, the surface tension of the water surface tends to act and bring the nearby bubbles to be attracted towards each other. Therefore, the bubbles are cluster together on the surface of the water.