An iron block sinks in water but floats on mercury because of differences in density and the principles of buoyancy.
Understanding Density and Buoyancy
The key to understanding this phenomenon lies in two core concepts:
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Density: Density is a measure of how much mass is contained within a given volume. A substance with a higher density has more mass packed into the same space.
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Buoyancy: Buoyancy is the upward force exerted by a fluid (liquid or gas) that opposes the weight of an immersed object. This force is equal to the weight of the fluid that the object displaces (Archimedes' principle).
Iron in Water
- The density of iron is approximately 7.87 g/cm³.
- The density of water is approximately 1.0 g/cm³.
Since iron is significantly denser than water, the weight of the iron block is greater than the buoyant force exerted by the water it displaces. Consequently, the iron block sinks. Simply put, for a given volume, the iron weighs more than the water.
Iron in Mercury
- The density of mercury is approximately 13.6 g/cm³.
- The density of iron is approximately 7.87 g/cm³.
Because mercury is more dense than iron, the weight of the mercury displaced by the iron block is greater than the weight of the iron block itself. This creates a buoyant force strong enough to support the iron, causing it to float. Again, the mercury is heavier for the same volume occupied by the iron.
Summary Table
| Substance | Density (g/cm³) | Iron's Behavior | Explanation |
|---|---|---|---|
| Water | 1.0 | Sinks | Iron's density (7.87 g/cm³) is greater than water's density (1.0 g/cm³). The buoyant force is insufficient to support the iron's weight. |
| Mercury | 13.6 | Floats | Iron's density (7.87 g/cm³) is less than mercury's density (13.6 g/cm³). The buoyant force is sufficient to support the iron's weight. |
In conclusion, whether an object sinks or floats depends on its density relative to the density of the fluid it is placed in. If the object is denser than the fluid, it sinks; if it's less dense, it floats.
