Small pieces of camphor placed on the surface of water are observed to move about rapidly. Which physical phenomenon is mainly responsible for this motion on the water surface?

Introduction / Context:
When a few small pieces of camphor are placed gently on the surface of clean water, they often start moving around in a seemingly random and rapid way. This interesting observation can be explained using surface physics. The question checks whether you can identify which physical phenomenon at the water surface is primarily responsible for driving this motion, and it links basic concepts of surface tension and contact surface chemistry.

Given Data / Assumptions:

• Pieces of camphor are floating on the water surface.
• The camphor gradually dissolves or spreads over the surface.
• The motion is along the surface, not sinking or rising.
• The options include diffusion, viscosity, surface tension and capillarity.

Concept / Approach:
Camphor slowly dissolves and spreads over the water surface, changing the local surface tension. Regions where more camphor is present have slightly lower surface tension compared to cleaner water. Surface tension forces act along the interface, tending to pull fluid from regions of low surface tension towards regions of higher surface tension. This creates unbalanced tangential forces that push the camphor pieces around. The resulting motion is sometimes called the camphor boat effect and is an example of motion driven by surface tension gradients, not by bulk viscosity or simple diffusion alone.

Step-by-Step Solution:
Step 1: Recognise that the motion occurs at the water surface, not deep in the bulk of the liquid.Step 2: Note that camphor gradually spreads, altering surface properties like surface tension locally.Step 3: Remember that regions of lower surface tension near the camphor and higher surface tension away from it create a gradient.Step 4: This gradient in surface tension leads to tangential forces acting along the interface.Step 5: These forces pull the surrounding water and effectively push the camphor pieces, making them move rapidly.Step 6: Conclude that surface tension and its variation are the main reasons for the rapid motion.

Verification / Alternative check:
If the surface tension of water is reduced uniformly by adding surfactants, the characteristic motion of camphor pieces significantly changes or stops, supporting the idea that surface tension gradients are essential. Moreover, if the surface is contaminated or covered with a uniform layer of another substance, the effect also disappears. This supports the explanation based on surface tension, rather than diffusion or viscosity alone.

Why Other Options Are Wrong:
Diffusion refers to random molecular motion spreading camphor molecules throughout the water, but by itself it does not produce directed motion of solid pieces. Viscosity measures internal friction of the fluid and tends to resist motion rather than create it. Capillarity refers to rise or fall of liquids in narrow tubes and is not the relevant effect for camphor pieces moving freely on an open surface. Thus, these options do not fully explain the observed rapid surface motion.

Common Pitfalls:
Students may incorrectly attribute the motion to diffusion because camphor is dissolving, or to viscosity because the liquid is involved. However, diffusion is random at the molecular level and would not produce the coordinated movement of camphor pieces. Another mistake is to think of capillarity whenever a liquid surface is mentioned. To avoid these confusions, focus on the fact that unbalanced surface tension forces along the interface are what cause the camphor to move.

Final Answer:
The rapid motion of camphor pieces on water is mainly due to surface tension effects.