A piece of glass sinks in water but floats in mercury because its density is greater than that of water, yet less than that of mercury. This fundamental principle governs whether an object will sink or float in any given liquid.
Understanding Density and Buoyancy
At the heart of this phenomenon is the concept of density—a measure of how much mass is contained in a given volume. Simply put, a denser object packs more material into the same amount of space. The interaction between an object's density and the density of the fluid it's placed in determines its behavior.
When an object is submerged in a fluid, it experiences an upward force called buoyancy. This buoyant force is equal to the weight of the fluid displaced by the object.
- If an object's density is greater than the fluid's density, the object weighs more than the fluid it displaces, so it sinks.
- If an object's density is less than the fluid's density, the object weighs less than the fluid it displaces, so it floats.
- If an object's density is equal to the fluid's density, it will remain suspended within the fluid.
For a deeper dive into buoyancy, explore resources like the Navegavation website on Buoyancy and Density.
Density Comparison: Glass, Water, and Mercury
Let's examine the typical densities of these three substances:
| Substance | Approximate Density (g/cm³) |
|---|---|
| Water | 1.0 |
| Glass | 2.4 – 2.8 |
| Mercury | 13.6 |
(Note: The density of glass can vary slightly depending on its composition.)
1. Glass in Water: Sinking
As the table shows, a typical piece of glass has a density ranging from 2.4 to 2.8 g/cm³, which is significantly greater than the density of water (1.0 g/cm³). Because glass is denser than water, it displaces a weight of water that is less than its own weight, causing it to sink.
2. Glass in Mercury: Floating
Conversely, the density of glass is much less than the density of mercury (13.6 g/cm³). When a piece of glass is placed in mercury, it displaces a weight of mercury that is greater than its own weight. This strong buoyant force from the very dense mercury lifts the glass, allowing it to float on the surface.
Practical Implications and Everyday Examples
This principle isn't just limited to glass, water, and mercury. It's why:
- Boats float: Even though a boat is made of dense materials like steel, its overall average density (including the air inside its hull) is less than that of water, allowing it to float.
- Icebergs float: Ice is less dense than liquid water, which is why only about 10% of an iceberg is visible above the surface.
- Hot air balloons rise: The hot air inside the balloon is less dense than the cooler air outside, creating lift.
Understanding the relative densities of substances helps us predict their behavior in various fluids and underpins many engineering and scientific applications.
