Which of the following phenomena is responsible for the spherical shape of soap bubbles?

Step 1: Analyze the question We need the phenomenon that makes soap bubbles spherical.

Step 2: Explain the concept Molecules in a liquid attract one another through cohesive forces. Molecules inside the liquid are pulled equally in all directions. Molecules at the surface are pulled only inward and sideways, so the surface behaves like a stretched elastic skin. This effect is called surface tension.

Surface tension makes the liquid surface contract to the smallest possible surface area for a given volume. A sphere has the smallest surface area for a given volume of any shape. That is why soap bubbles (and small liquid drops) are spherical.

Step 3: Check each option

• A. Adhesion: attraction between unlike molecules, such as water and glass. It is not responsible for the bubble's shape.
• B. Capillarity: the rise or fall of liquid in narrow tubes. It results from surface tension and adhesion, but it is not the cause of bubble shape.
• C. Cohesion: attraction between like molecules. It is the underlying force, but the phenomenon that shapes the surface is surface tension. Cohesion alone does not explain the surface contracting to a minimum area.
• D. Osmosis: movement of solvent through a semipermeable membrane. It is irrelevant here.
• E. Surface tension: the film tries to minimize its area, producing a sphere. ✔

Step 4: Answer The correct option is E. Surface tension.

Exam tip: Remember "small area, sphere, surface tension." Whenever a question mentions the shape of drops or bubbles, or insects walking on water, think surface tension. Think capillarity for liquid rising in narrow tubes, such as water in a thin tube or oil in a wick.