Objects less dense than water float. Objects denser than water sink.
Rearranged: mass = density × volume. Volume = mass / density.
Density depends on: what the material is made of AND how tightly particles are packed.
Measuring density — regular objects
Measure mass with a balance (g or kg).
Measure dimensions (l, w, h for cuboid; d for sphere/cylinder) with ruler/vernier callipers.
Calculate volume from formula (e.g. V = l × w × h for cuboid).
Calculate density: ρ = m/V.
Convert units consistently: all in cm → g/cm³; all in m → kg/m³.
Measuring density — irregular objects
Measure mass with balance.
Eureka can (displacement method): fill can to spout level with water. Submerge object. Collect displaced water. Volume of displaced water = volume of object.
Alternatively: measure water level in measuring cylinder before and after submerging.
Use density = mass / volume.
Particle model and density
Solids: particles closely packed in regular lattice → high density.
Liquids: particles close but randomly arranged, slightly further apart → slightly lower density than solid (usually).
Gases: particles far apart, random motion → very low density.
Water: unusual — ice is less dense than liquid water (particles in ice lattice have more space) → ice floats.
Different materials have different densities depending on atomic mass and spacing.
A metal block has mass 270 g and dimensions 3 cm × 5 cm × 2 cm. Calculate its density in g/cm³ and identify the metal. (Density of aluminium = 2.7 g/cm³, iron = 7.9 g/cm³) (3 marks)
Hint: Step 1: V = l × w × h. Step 2: ρ = m/V. Step 3: compare to given densities.
+30 XP
1 markWhy objects floatAQA 8463 style
A block of wood (density 600 kg/m³) is placed in water (density 1000 kg/m³). What happens and why?
A It sinks because wood is denser than most materials
B It floats because its density is less than water
C It dissolves in the water
D It sinks because water exerts pressure on it
3 marksIrregular object methodAQA 8463 P3 style
Describe how you would find the density of an irregular stone using a measuring cylinder and a balance. (3 marks)
Hint: Two steps: (1) mass from balance. (2) volume from displacement in measuring cylinder. Then density = mass/volume.
+30 XP
1 markParticle explanationAQA 8463 style
Why are gases much less dense than solids?
A Gas particles are lighter than solid particles
B Gas particles move faster and so spread out
C Gas particles are much further apart than solid particles, so the same mass occupies a much larger volume