Density
📖Definition
Density is mass per unit volume:
ρ = m / V
- ρ (rho) = density in kg/m³ (or g/cm³).
- m = mass in kg (or g).
- V = volume in m³ (or cm³).
The units must match. 1 g/cm³ = 1000 kg/m³.
Typical values
| Material | Density (kg/m³) |
|---|---|
| Water | 1000 |
| Aluminium | 2700 |
| Iron | 7900 |
| Lead | 11 300 |
| Air (sea level) | ≈ 1.2 |
Solids and liquids are roughly 1000× denser than gases because their particles are much closer together. Gases compress easily because there is empty space between particles; solids and liquids do not.
Density and particle model
| Phase | Particle arrangement | Density |
|---|---|---|
| Solid | Tightly packed in a regular pattern | High |
| Liquid | Close but free to move | Slightly lower than solid (water is unusual) |
| Gas | Far apart, fast random motion | Very low |
Measuring density of a regular solid
- Use a balance to measure mass.
- Use a ruler to measure dimensions; calculate volume (e.g. cube V = l³, cuboid V = l × w × h).
- Calculate ρ = m / V.
Core Practical (Paper 2): density of an irregular solid (displacement method)
- Measure mass on a balance.
- Half-fill a measuring cylinder with water; record initial volume V₁.
- Lower the object in fully without splashing; record new volume V₂.
- Volume of object = V₂ − V₁.
- Calculate ρ = m / (V₂ − V₁).
A Eureka can offers an alternative — water displaced flows into a measuring cylinder.
Density of a liquid
- Measure mass of an empty measuring cylinder (m₁).
- Pour a known volume of liquid (V) into it.
- Measure mass with liquid (m₂).
- ρ = (m₂ − m₁) / V.
Edexcel exam tip
For "find the density of an irregular stone" the marks fall: balance for mass M1; displacement method to find volume M1; ρ = m / V A1. Drawing a diagram earns no extra marks; correctly stating the equation and calculation does.
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