What Is Density?
Density is the amount of mass in a given volume. A compact, heavy material is dense; a light, spread-out material is less dense.
The unit is usually g/cm³ or kg/m³.
For example:
- Water: 1.0 g/cm³
- Iron: 7.9 g/cm³
- Air: ~0.0013 g/cm³
Density and States of Matter
Particle theory explains why different states have different densities:
- Solids — particles closely packed, regular arrangement → high density
- Liquids — particles close but disordered, slight gaps → similar density to solid (slightly less)
- Gases — particles far apart, moving freely → very low density (about 1000× less than liquid)
Ice floats on water because ice is slightly less dense than liquid water (0.92 g/cm³ vs 1.0 g/cm³). This is unusual — most solids are denser than their liquid form. It happens because ice has a more open crystal structure.
Floating and Sinking
Whether an object floats or sinks depends on its density compared to the fluid it is placed in:
- Denser than the fluid → sinks
- Less dense → floats
Example: wood (0.4–0.9 g/cm³) floats in water; steel (7.9 g/cm³) sinks. A steel ship floats because its hollow hull is filled with air, making its average density less than water.
Calculating Density
Example: A block of metal has a mass of 270 g and a volume of 100 cm³. What is its density?
This is close to the density of aluminium (2.7 g/cm³), so the block is likely aluminium.
Real world: Ice floating in the ocean
Icebergs float because ice (0.92 g/cm³) is less dense than sea water (~1.03 g/cm³). About 90% of an iceberg is submerged, but the top remains above water. This unusual property of water — being less dense as a solid — is vital for aquatic life: ice forms at the surface, insulating the liquid water below.
Real world: Hot air balloons
Heating the air inside a hot air balloon makes the air expand and become less dense than the cooler air outside. Because the overall density of the balloon (including the basket and envelope) becomes less than the surrounding air, it rises. This is density at work at a very large scale.
Real world: Steel ships floating
A solid lump of steel sinks in water (density 7.9 g/cm³ vs water 1.0 g/cm³). But a steel ship floats because it is hollow — the hull is filled with air. The average density of the ship (steel + trapped air) is less than water, so it floats. If the hull fills with water, the average density increases and the ship sinks.
Where do you see this in real life? Oil floats on water because it is less dense. Ice floats in your drink. Hot air rises in a room. All of these are density differences in action every day.