The Particle Model & Density (Years 7–11) — Science

Using the particle model to explain density — mass ÷ volume — and floating.

Pick up a bag of crisps, then pick up a brick the same size. Same volume, wildly different feel. That difference is density — how much mass is crammed into a given volume.

Why this matters: Density explains everyday puzzles — why ships made of steel float, why ice bobs on a drink, why oil sits on top of water. It also lets you identify a material without weighing a huge lump: measure a small piece and the density is the same.

Same size, but the right cube has more mass packed in

What density really means

Every material is made of tiny particles. Density is a measure of how tightly those particles are packed and how heavy each one is. Think of it like a car park: a small car park crammed bumper-to-bumper has a high "car density"; a huge empty one has a low car density even if it holds a few cars.

The equation is:

Density = mass ÷ volume
Mass is measured in grams (g) or kilograms (kg); volume in cm³ or m³. So density comes out in g/cm³ or kg/m³.

Cover the quantity you want: D = mass ÷ V.

The three states, ranked

Same box: packed particles (solid) mean more mass, so higher density.

The same substance changes density when it changes state, because the particle spacing changes.

State	Particle spacing	Density
Solid	Packed tight, touching	Highest
Liquid	Touching but sliding	High
Gas	Far apart, mostly empty space	Lowest

This is why squashing a gas into a smaller volume increases its density — you keep the same mass of particles but give them less room.

Floating and sinking

Less dense than water → floats; denser → sinks.

Whether something floats depends only on comparing its density with the liquid's. An object floats if it is less dense than the liquid, and sinks if it is denser. Water is about 1 g/cm³, so anything below that floats on water and anything above sinks.

Wood (~0.6 g/cm³) floats.
Iron (~7.9 g/cm³) sinks.
Ice (~0.92 g/cm³) floats — even though it is a solid.

Worked reasoning

Worked example. A block has mass 60 g and volume 20 cm³. Find its density.
Ask: "how much mass per one cm³?" That is mass ÷ volume.
Density = 60 ÷ 20 = 3 g/cm³.
Since 3 g/cm³ is greater than water's 1 g/cm³, this block would sink.

Common mix-up: "Heavy things sink." Not true on their own. A huge steel ship is very heavy but floats, while a tiny steel bolt sinks. What matters is density (mass per volume), not total mass. The ship's hull traps air, so the whole ship's average density is less than water.

Memory hook: Density = Mass over Volume — picture "DMV", and remember mass sits on top of the division.