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.