Density

Density tells you how much mass is packed into a given volume.

ρ = m / V (kg/m³ or g/cm³)

To convert: 1000 kg/m³ = 1 g/cm³.

Why solids are denser than gases

In a solid, particles are packed tightly in a regular arrangement and vibrate about fixed positions. The mass per volume is high. In a liquid, particles are still close but disordered and free to slide past each other. Density is similar to (slightly less than) the solid. In a gas, particles are far apart, moving freely at high speed. Density is much lower than for solids and liquids.

Measuring density

Regular solid: measure mass on a top-pan balance; calculate volume from dimensions (l × w × h). Irregular solid: mass on balance; volume by displacement — fill a measuring cylinder with water, record V₁, lower the object into the cylinder, record V₂. V = V₂ − V₁. Liquid: mass an empty measuring cylinder, add the liquid, mass again. Volume read from the scale.

Floating and sinking

An object floats on a liquid if it is less dense than the liquid. A wooden block (≈ 600 kg/m³) floats on water (1000 kg/m³); an iron nail (7800 kg/m³) sinks.

CCEA tip

Look for the units the question expects in the answer. If the data is in cm and g, your answer is in g/cm³ — converting to kg/m³ unnecessarily wastes time and risks an arithmetic slip that costs the A1 mark.