Matter: Density of Materials

Definition of Density

Density is a measure of how much mass is contained in a given volume.
It is a scalar quantity (it only has magnitude, not direction) and is expressed in the units kg/m^3 (kilograms per cubic metre).
Both mass and volume contribute to the density of an object or material.
Even if two objects are the same size, the one with more mass (more matter in it) will have a greater density.

The formula to calculate density is: Density (ρ) = Mass (m) ÷ Volume (V)
You have to know both the mass and volume of the object to find out its density.
To calculate the mass of an object, you can use a balance.
To calculate the volume of a regular-shaped solid, you can use measurements of its dimensions and geometrical formulas.
For irregular-shaped objects or fluids, you can use a displacement vessel or eureka can to determine volume by displacement.

Density is a key factor which influences whether an object will float or sink in a fluid.
An object will float in a fluid if it is less dense than the fluid, and it will sink if it is denser.
The same principle explains why hot air rises and cool air drops; when a gas is heated, it expands and its density decreases, allowing it to rise. When cooled, the gas contracts, its density increases, and it dropps.

The Particle Model can help explain why materials differ in density.
Solids are generally denser than liquids and gases, due to their closely packed particles.
In a solid, particles are tightly packed in a fixed arrangement, while in liquids and gases, the particles are more spread out, thus the object has a lower mass for the same volume.
Temperature can also affect the density of materials. When heated, the particles move apart, thus increasing the volume which effectively decreases the density.

Understanding the principle of density has important practical applications.
It helps in designing ships and submarines, decanting oil from water, hot air balloons and numerous other aspects of design, manufacturing and construction.