Density

Key Definition Density is mass per unit volume, defined by the equation ρ = m/V, where ρ is measured in kg m⁻³ (or g cm⁻³).

Because density is a property of the material rather than the size of an object, two objects made from the same substance always have the same density regardless of their shape or mass.

ρ = m/V

The equation shows that density is directly proportional to mass for a fixed volume and inversely proportional to volume for a fixed mass, so doubling the mass of a sample while keeping its volume constant doubles its density.

Three practical methods cover all the solid and liquid density determinations required by CIE; in every case the density is then calculated using ρ = m/V.

• The mass of the liquid is found by subtraction (mass of the cylinder with liquid minus mass of the empty cylinder).
• The volume is read from the measuring cylinder at the bottom of the meniscus.

Regularly shaped solid

• The mass is measured using a balance.
• The dimensions are measured with a ruler.
• The volume is calculated using the appropriate geometric formula.

Irregularly shaped solid

• The mass is measured using a balance.
• The volume is found by displacement — the rise in water level in a measuring cylinder equals the volume of the submerged solid.

Whether an object floats or sinks depends entirely on its density relative to the surrounding fluid: objects with a lower density than the fluid float, and those with a higher density sink.

Extended The same principle applies to immiscible liquids, where the less dense liquid always forms the upper layer.