IGCSE Physics / Density and pressure
Density revision notes
Edexcel International GCSE Physics (4PH1), chapter 18: Density and pressure, specification point 5.3, 5.1. Free lesson.
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Density is the mass packed into a given volume. You can calculate density or mass from ρ = m/V, convert between density units, and use density to explain why objects float.
What density measures
Density tells you how much mass is packed into each unit of volume. The equation is ρ = m/V, where ρ is the Greek letter rho. Match the mass and volume units so the density unit is correct. ρ is density, in kg/m³ or g/cm³. m is mass, in kg or g. V is volume, in m³ or cm³. Use kg with m³ to get kg/m³, or g with cm³ to get g/cm³.
How to calculate density
Write the equation first, then substitute the values. Dividing mass by volume gives the mass in each unit of space. Check the units before giving your answer.
How to convert density units
One cubic metre contains 1 000 000 cubic centimetres, and one kilogram contains 1000 grams. These changes mean that 1 g/cm³ = 1000 kg/m³. So the numerical value in kg/m³ is always bigger than the value in g/cm³. To convert g/cm³ to kg/m³, multiply by 1000. To convert cm³ to m³, divide by 1 000 000, because 1 m³ = 100 cm × 100 cm × 100 cm. For example, 250 cm³ = 250 ÷ 1 000 000 = 0.00025 m³, not 2.5 m³.
How to calculate mass
The density equation can be rearranged to find mass. Substitute the known density and volume, then multiply both sides by the volume.
Why solids and liquids have different densities
For the same substance, the particles are usually packed more tightly in a solid than in a liquid. The same mass therefore takes up less volume as a solid, so its density is usually higher. Water is the exception: ice is less dense than liquid water, so it floats. An object floats in a liquid if it is less dense than that liquid. In a solid, particles are tightly packed in a regular pattern. In a liquid, particles remain close together but are a little further apart on average. Water is the exception to the usual solid-and-liquid density pattern.