# 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.

## Determining the Density of a Material

• 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.

## Effects of Density

• 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.

## Density and the Particle Model

• 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.

## The Importance of Density in Everyday Life and Industry

• 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.