When a small steel ball is gently placed on the surface of liquid mercury, it does not sink. What is the main reason for this behaviour?

Introduction / Context:
This question combines ideas from fluid mechanics and properties of materials. Mercury is a very dense liquid metal, and steel is a dense solid. Intuitively, many people expect solids to sink in liquids, but whether an object sinks or floats actually depends on density and buoyancy, not on whether the object is solid or liquid. Observing that a steel ball can float on mercury under appropriate conditions is a striking demonstration of Archimedes principle. Understanding why this happens helps you connect density concepts with real examples.

Given Data / Assumptions:

• A small steel ball is placed gently on the surface of mercury.
• Mercury is a dense liquid with a density much greater than that of water.
• Steel also has significant density, greater than water, but less than mercury.
• We assume the ball is not dropped violently, so splashing effects are avoided.

Concept / Approach:
According to Archimedes principle, a body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced. If the fluid is very dense, a relatively small volume of fluid is enough to support a given weight. Mercury has a density around 13.6 times that of water, while steel is roughly 7 to 8 times as dense as water. Because mercury is denser than steel, a steel ball can displace a volume of mercury whose weight equals the weight of the ball before it completely submerges. As a result, the ball floats partially immersed. Surface tension effects can contribute for very small objects, but the primary reason in this classic example is the density difference and buoyant force.

Step-by-Step Solution:
Step 1: Recall that whether an object sinks or floats depends on the comparison between its density and the density of the fluid. Step 2: Note that steel is denser than water and therefore sinks in water, but mercury is much denser than both water and steel. Step 3: When the steel ball is placed on mercury, it displaces some mercury. The buoyant force depends on the weight of displaced mercury. Step 4: Because mercury is very dense, even a modest displacement provides a buoyant force that can balance the weight of the steel ball. Step 5: At equilibrium, the upward buoyant force equals the weight of the ball, so it floats instead of sinking, demonstrating that mercury density exceeds steel density.

Verification / Alternative check:
Imagine replacing mercury with water while keeping the same steel ball. Steel has greater density than water, so in water the ball displaces a volume whose weight is less than the ball's own weight before it becomes fully submerged, and thus it sinks. The different outcomes in mercury versus water illustrate that it is the density of the fluid compared to the density of the solid that matters. Reference tables of densities confirm that mercury has higher density than steel, matching the explanation and supporting the correct option.

Why Other Options Are Wrong:
Surface tension of mercury does affect very small particles, but for a steel ball of ordinary size, buoyancy from high density is the main cause of floating in textbook treatments. Mercury is not a semisolid; it is a true liquid metal at room temperature with well defined fluid properties. High viscosity would only slow movement; it does not by itself prevent sinking in the long run if density conditions favour sinking.

Common Pitfalls:
Students sometimes attribute all floating effects on liquids like mercury to surface tension, especially if they have seen small objects resting on surfaces without sinking. While surface tension can be significant for microscopic particles, density and buoyancy are the dominant factors for larger objects like steel balls in typical classroom demonstrations. To avoid confusion, always check density comparisons first when explaining why objects float or sink.

Final Answer:
A steel ball placed on mercury does not sink mainly because the density of mercury is greater than the density of steel, so buoyant force can balance its weight.