Materials: Density

Materials: Density

Key Concepts of Density

  • Density is a measure of mass per unit volume of a substance.
  • It tells us how ‘packed’ a substance is and is a property characteristic to the material.
  • It is represented by the Greek letter ‘rho’ (ρ), measured in kilograms per cubic metre (kg/m³), though it is also often expressed in g/cm³.

Calculating Density

  • The formula to calculate density is: Density = Mass / Volume
  • This means density (ρ) equals the mass (m) of the substance divided by its volume (V).
  • Rearranging the formula, you could also find mass by multiplying density and volume (Mass = Density x Volume), or find volume by dividing the mass by the density (Volume = Mass / Density).

Density and States of Matter

  • The state of matter of a substance also affects its density.
  • On average, solids are denser than liquids, and liquids denser than gases, due to the differences in the arrangement and movement of their particles.
  • However, there are exceptions, such as ice being less dense than water, causing it to float.

Applies to Phases of Matter

  • The density of a substance changes with phase changes.
  • For instance, when water freezes into ice, its density decreases. When ice melts into water, its density increases.
  • Density of gases increases with increasing pressure and decreases with increasing temperature, following the ideal gas law, which states, PV=nRT, where P is pressure, V is volume, n is the number of molecules, R is the gas constant, and T is temperature.

Density in Everyday Life

  • Understanding density explains many everyday phenomena and is used in a wide range of applications.
  • For example, hot-air balloons rise because hot air is less dense than cooler air.
  • Another example is oil floats on water as it has a lesser density.
  • In geology, different layers of rocks can be identified based on their densities.

Relative Density

  • Relative density, also known as specific gravity, is the ratio of the density of a substance to a reference substance.
  • This is most commonly water for liquids, and air for gases.
  • Fluids with a relative density less than 1 will float in the reference substance, whereas fluids with a relative density more than 1 will sink.
  • Relative density is a dimensionless quantity, meaning it does not have physical units.