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11.2Chapter 11: Patterns in the Periodic Table

Explaining density — Alhussan Chemistry - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 7 2 quizzes

📚 Alhussan Chemistry - Stage 9 Hub / Topical Revision
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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 8What density depends on

Two factors
  • 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
  • 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 8Mass of the particles

Mass of the particles
Rule
  • Rule in the solid state, substances with the heaviest particles have the greatest densities.
Metals
  • Metals with the same type of packing, the metal with heavier atoms is denser.
MetalRelative mass of atomsDensity (g/cm3)
Magnesium241.8
Aluminium272.7
Gold19719
Mass of the particles
Comparison
  • Comparison gold atoms are about seven times as massive as aluminium atoms, and gold is about seven times denser.
Prediction
  • Prediction tungsten (atom mass 184) is predicted to be denser than chromium (atom mass 52).

Section 3 of 8Particle packing in the three states

StateParticle spacingDensity
SolidVery close, regular patternHigh
LiquidClose, random arrangementSlightly lower or similar
GasFar apartVery low
Particle packing in the three states
Liquid and gas
  • 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
  • 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
  • Gas density a gas is about a thousand times less dense than a liquid.

Section 4 of 8Changes of state and water

Melting and boiling
  • Melting and boiling the mass is unchanged but the volume usually increases, so the density falls.
Water
  • 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
  • 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 8The dried-peas analogy

Analogy
Analogy a comparison between one thing and another that helps to explain something.
Liquid model
  • 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
  • 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
  • Strength it helps to explain density.
Limitation
  • Limitation you need to move the jar to move the peas, whereas real particles move themselves.

Section 6 of 8Everyday uses of density ideas

Floating and sinking
  • 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
  • Oil it floats on water because it is less dense, so oil spills spread over the sea surface.
Balloons
  • 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
  • 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
  • Earth's layers a dense iron-rich core lies at the centre, surrounded by a less dense mantle and a thin, light crust.
Oceans
  • Oceans cold salty water is denser than warm fresh water and sinks, driving currents.

Section 7 of 8Key terms

TermMeaning
Densitythe mass per unit volume.
Particle modela model that describes matter as tiny moving particles.
Packinghow closely particles are arranged.
Open structurean arrangement with gaps between molecules.
Analogya comparison that helps to explain something.
Relative massthe mass of an atom compared with other atoms.

Section 8 of 8Exam technique

1
Saying that particles themselves expand on heating — the spacing between particles increases.
2
Saying that gas particles weigh less than solid particles of the same substance — they are the same, only more widely spaced.
3
Forgetting that mass is conserved when a substance changes state — only the volume changes.
4
Saying that ice is denser than water — ice is less dense, so it floats.

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