Density and States of Matter

Density

density = mass ÷ volume      ρ = m / V

Units: kg/m³ or g/cm³. An object floats if it's less dense than the fluid.

Worked example

A block of mass 240 g and volume 30 cm³:

ρ = 240 / 30 = 8 g/cm³

Measuring density

• Regular solid: measure mass; calculate volume from dimensions.
• Irregular solid: use a displacement (eureka) can — volume = water displaced.
• Liquid: measure mass of a known volume in a measuring cylinder.

The three states

State	Arrangement	Movement	Energy
Solid	Fixed, close, regular	Vibrate in place	Lowest
Liquid	Close, random	Flow around each other	Medium
Gas	Far apart, random	Fast, all directions	Highest

Changes of state

Physical changes — mass is conserved and they're reversible; only the arrangement and energy of particles change (not the particles themselves).

• melting / freezing, boiling / condensing, sublimation.

Exam tip

Changes of state are physical, not chemical — the substance stays the same, so mass is conserved.