CHEMISTRY · Stage 9 · Chapter 11: Patterns in the Periodic Table

Chemistry 11.2 Explaining density — Particle Mass and Packing in Solids, Liquids and Gases

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11.2 Explaining density — Particle Mass and Packing in Solids, Liquids and Gases — Alhussan International Academy — Chemistry (Stages 7–9) notes: Explain why substances differ in density and why a substance has different densities in its three states, using the particle model and…

Two factors
Layers of liquids
Comparison
Prediction
Liquid and gas
Solid and liquid

Section 1 of 6What density depends on

Two factors
- the density of a substance depends on the mass of its particles and on how closely packed the particles are.
Layers of liquids
- honey (bottom), milk and oil (top) form separate layers because their densities differ, with honey the densest and oil the least dense.

Section 2 of 6Mass of the particles

Rule
- in the solid state, substances with the heaviest particles have the greatest densities.
Metals
- with the same type of packing, the metal with heavier atoms is denser.

Metal	Relative mass of atoms	Density (g/cm³)
Magnesium	24	1.8
Aluminium	27	2.7
Gold	197	19

Comparison
- gold atoms are about seven times as massive as aluminium atoms, and gold is about seven times denser.
Prediction
- tungsten (atom mass 184) is predicted to be denser than chromium (atom mass 52).

Section 3 of 6Particle packing in the three states

State	Particle spacing	Density
Solid	Very close, regular pattern	High
Liquid	Close, random arrangement	Slightly lower or similar
Gas	Far apart	Very low

Liquid and gas
- particles in a liquid are more closely packed than in a gas, so the liquid has a greater density.
- 500 cm³ of steam has a smaller mass than 500 cm³ of liquid water because the particles are further apart in steam.
Solid and liquid
- for most substances the solid is a little denser than the liquid because its particles are a little more closely packed.
Gas density
- a gas is about a thousand times less dense than a liquid.

Section 4 of 6Changes of state and water

Melting and boiling
- the mass is unchanged but the volume usually increases, so the density falls.
Water
- it is unusual because ice is less dense than liquid water, since the molecules form an open structure when they freeze.
- This is why ice floats and pipes can burst when water freezes inside them.
Heating a gas
- the particles move faster and spread out, so a given mass occupies a larger volume and the density falls.
- This is why warm air rises in a hot-air balloon.

Section 5 of 6The dried-peas analogy

Analogy = a comparison between one thing and another that helps to explain something.
Liquid model
- peas in a jar touch each other without a pattern and slide randomly over each other when the jar is moved gently.
Gas model
- shaking the jar hard moves the peas far apart.
- A volume of 100 cm³ in the gas model has fewer particles, and so less mass, than in the liquid model, so the gas has a lower density.
Strength
- it helps to explain density.
Limitation
- you need to move the jar to move the peas, whereas real particles move themselves.

Section 6 of 6Everyday uses of density ideas

Floating and sinking
- wood floats in water because its average density is lower, and a steel ball sinks because steel is denser.
- Iron floats on mercury because mercury has a very high density.
Oil
- it floats on water because it is less dense, so oil spills spread over the sea surface.
Balloons
- hot-air balloons rise because heated air is less dense, and helium balloons float because helium is less dense than air.
Divers and life jackets
- weight belts increase a diver's average density so they sink, and life jackets with air or foam reduce it so the wearer floats.
Earth's layers
- a dense iron-rich core lies at the centre, surrounded by a less dense mantle and a thin, light crust.
Oceans
- cold salty water is denser than warm fresh water and sinks, driving currents.

Last updated: 13 October 2026