Density of matter
Density is the mass of a substance per unit of volume. It is found by dividing mass by volume: ρ = m : V, where ρ is density, m is mass and V is volume. If mass is in grams (g) and volume in cubic centimetres (cm³), density comes out in g/cm³, the mass in grams of each 1 cm³. The SI unit is kg/m³; 1 g/cm³ = 1000 kg/m³. For example water has a density of 1 g/cm³ (1000 kg/m³), iron 7.8 g/cm³, aluminium 2.7 g/cm³, gold 19.3 g/cm³ and cork 0.24 g/cm³. Equal copper and aluminium cubes differ in mass: copper’s is larger because copper is denser. An object less dense than the liquid floats; a denser one sinks. Ice (about 0.9 g/cm³) is less dense than water, so it floats.
Experiment: take two identical mandarins and peel one. Lower both carefully into glasses of water: the unpeeled mandarin floats (air pockets in the peel make it less dense) and the peeled one sinks (it is denser than water). Explain with density. Safety: pupils with citrus allergies should not handle them; wash fruit before eating; wipe up spilt water.