1. Density explains why objects of the same size can weigh differently

A small block of iron can have more mass than a much larger block of wood. This is because iron has a much higher density than most types of wood.

2. Density helps determine whether an object floats

An object generally floats in a liquid when its average density is lower than the liquid's density. A wooden block floats in water because its average density is usually lower than water's.

3. Ice is less dense than liquid water

Most substances become denser when they freeze, but water behaves unusually. Ice has a more open crystal structure, so it floats on liquid water. This helps aquatic life survive beneath frozen lakes.

4. Liquids can form separate layers according to density

Honey is generally denser than water, while many cooking oils are less dense. When carefully poured together, these liquids can form layers, although mixing and temperature can affect the result.

5. Hot air is usually less dense than cold air

Heating air at approximately constant pressure makes it expand, reducing its density. Hot-air balloons rise because the surrounding cooler air provides an upward buoyant force.

6. Gold is much denser than aluminum

Gold has a density of approximately 19.3 g/cm³, while aluminum has a density of approximately 2.7 g/cm³ at room temperature. Equal volumes of gold therefore have about seven times the mass of aluminum.

7. Saturn has a lower average density than water

Saturn's average density is about 0.69 g/cm³, less than water's roughly 1 g/cm³. The familiar claim that Saturn could float in water is a thought experiment—there is no ocean large enough to hold the planet.

8. Earth becomes denser toward its center

Earth's average density is about 5.5 g/cm³, much higher than the density of most surface rocks. Its dense iron-rich core contributes significantly to this high average.

9. Density can change with temperature and pressure

Many materials expand when heated, reducing their density if their mass stays constant. Increasing pressure often increases density, particularly in gases and compressible materials.

10. Density can help identify unknown substances

By measuring a sample's mass and volume, scientists can calculate its density and compare the result with reference values. This is useful in laboratories, manufacturing, and material testing.

11. Helium balloons rise because of buoyancy

Helium is less dense than the surrounding air. The upward buoyant force can exceed the combined weight of the helium and balloon, causing it to rise.

12. Porous materials can have very different densities

A sponge or foam contains many air spaces. Its bulk density, measured using its overall volume, can be much lower than the density of the solid material forming it.

13. Submarines control their buoyancy by changing average density

Submarines take water into ballast tanks to increase their average density and descend. They expel water using compressed air to reduce their average density and rise.

14. Some stars have extraordinary densities

White dwarfs pack roughly a Sun's worth of mass into a volume comparable to Earth's. Their matter is so compressed that a small volume can have a mass of many tonnes or more.

15. Density is important in separating materials

Density differences help engineers separate substances in mining, recycling, water treatment, and food processing. Separation methods may also use particle size, buoyancy, or centrifugal forces.