Q4. Water would rise 5.9 cm in a capillary of radius 0.25 mm, but the tube sticks out only 3.0 cm above the water. What happens?
The water rises to the top of the tube and stops; it does not overflow. The meniscus becomes flatter, its radius of curvature growing (to about 0.25 × 5.9/3.0 ≈ 0.49 mm) so that the smaller pressure difference just supports the 3.0 cm column.
Q5. Two soap bubbles, of radii 3.0 cm and 4.0 cm, are joined by a tube. Which way does the air flow?
The excess pressure 4S/r is larger in the smaller bubble (by a factor 4/3), so air flows from the 3.0 cm bubble into the 4.0 cm one: the small bubble shrinks and the large one grows.
Q6. Why does adding detergent help water clean a greasy cloth?
Detergent lowers the surface tension of water and its angle of contact with the fibres, so the water wets the cloth and soaks in. Detergent molecules also attach to the grease by their oil-loving ends and surround it in small droplets, which are carried away in the rinse.