Density
- Density is the mass per unit volume of a material: ρ = m ÷ V.
- Density is measured in kg/m³ (or g/cm³). 1 g/cm³ = 1000 kg/m³.
- To find mass: m = ρ V. To find volume: V = m ÷ ρ.
Density and the particle model
- In solids, particles are closely packed in a regular arrangement, so solids are usually the densest state.
- In liquids, particles are close together but randomly arranged, so liquids are usually slightly less dense than solids.
- In gases, particles are far apart, so gases have a much lower density.
- Water is an unusual exception: ice is less dense than liquid water, so ice floats.
Required practical: regular solids
- Measure the mass of the object with a balance.
- Measure its length, width and height with a ruler and calculate the volume (length × width × height for a cuboid).
- Calculate the density: ρ = m ÷ V.
Required practical: irregular solids and liquids
- For an irregular object: fill a eureka (displacement) can with water, lower in the object, and collect the water that overflows in a measuring cylinder. Its volume equals the volume of the object.
- For a liquid: measure the mass of an empty measuring cylinder, add a known volume of liquid, and measure the mass again. The difference is the mass of the liquid.
- Read measuring cylinders at eye level, from the bottom of the meniscus.
Key terms
- Density
- Mass per unit volume: ρ = m ÷ V.
- Mass
- The amount of matter in an object, measured in kilograms.
- Volume
- The amount of space an object takes up, measured in m³ or cm³.
- Eureka can
- A container with a spout used to measure the volume of an irregular object by displacement.
- Displacement
- Water pushed out of the way by an object, equal to the object's volume.
- Meniscus
- The curved surface of a liquid in a measuring cylinder.