Density and the particle model

Density is mass per unit volume: ρ = m ÷ V, measured in kg/m³ or g/cm³. Water has a density of 1 g/cm³ (1000 kg/m³). Objects less dense than water float, and denser objects sink. The particle model explains density: in solids particles are closely packed, so most solids are dense; in gases particles are far apart, so gases have very low density.

To find the density of a regular solid, measure its mass on a balance and calculate its volume from its dimensions. For an irregular object, use displacement: lower it into a measuring cylinder or a eureka can and measure the volume of water pushed out. Changes of state are physical changes: mass is conserved and the material recovers its properties if the change is reversed.