Density and the particle model
Mass in every cubic metre
Density tells you how much mass is packed into each unit of volume.
density = mass ÷ volume
ρ = m ÷ V
Mass in kg, volume in m³, density in kg/m³ (AQA: recall this one; Edexcel and OCR: it is on the list you learn too). In the lab, grams and cm³ give g/cm³.
- A 0.54 kg block has a volume of 0.0002 m³
- ρ = 0.54 ÷ 0.0002
- ρ = 2700 kg/m³ (it is probably aluminium)
Why gases are so light
The particle model explains the numbers:
- Solid: particles touching, in a regular pattern, vibrating about fixed positions. High density.
- Liquid: particles touching but jumbled, sliding past each other. Similar density to the solid.
- Gas: particles far apart, moving fast in random directions. Density about a thousand times lower.
Changing state does not change the particles, only how they are arranged, so the mass stays the same.
g/cm³ and kg/m³
One cubic metre is 100 × 100 × 100 = 1 000 000 cm³, and one kilogram is 1000 g.
- 1 g/cm³ = 1000 kg/m³
- Water: 1.0 g/cm³ = 1000 kg/m³
- Aluminium: 2.7 g/cm³ = 2700 kg/m³
Anything less dense than water floats in it. Compare densities in the same unit before you decide.
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