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1.1Principles of Chemistry

States of Matter — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 7 parts Lesson 7 of 25 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

States of Matter: Changes of State, Diffusion, Solutions and Solubility — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Describe the three states, the changes between them, diffusion, solution terms and how to use solubility curves.The three states of matter are solid, liquid and gas.

  • Evaporation
  • Condensation
  • Sublimation
  • Worked example 1
  • Worked example 2
  • Worked example 3

Part 1 of 5The three states and particle model

The three states of matter are solid, liquid and gas. A substance can usually exist in all three depending on temperature and pressure. In the simple model particles are small solid spheres.

SolidLiquidGas
ArrangementRegularRandomRandom
MovementVibrate about fixed positionMove around each otherMove quickly in all directions
ClosenessVery closeCloseFar apart

The properties of single atoms are not the same as those of bulk matter, for example an atom is not itself "solid".

Part 2 of 5Changes of state

The energy needed for a change of state depends on the strength of the forces between particles: stronger forces mean higher melting and boiling points. Changes of state are physical changes; the particles themselves stay the same.

Melting
Melting solid to liquid at the melting point; absorbed energy becomes kinetic energy so particles vibrate more and start to flow.
Boiling
Boiling liquid to gas at the boiling point; bubbles form throughout the liquid.
Freezing
Freezing liquid to solid at the same temperature as melting (water 0°C), by loss of thermal energy.
Evaporation
Evaporation occurs over a range of temperatures, only at the surface, where high-energy particles escape; faster with larger surface area and warmer liquid.
Condensation
Condensation gas to liquid when cooled; particles lose energy and group together.
Sublimation
Sublimation solid directly to gas (iodine, solid carbon dioxide); the reverse is deposition.

Part 3 of 5Diffusion and dilution

Diffusion in gasesred-brown bromine gas spreads from one gas jar to another within minutes because the particles move randomly with large gaps.

Diffusion and dilution
In liquids: potassium manganate(VII) spreads in water more slowly because particles are close and slower.
Dilution of potassium manganate(VII) solution many times, with the colour fading only slowly, shows there are very many tiny particles.
These experiments are evidence for the kinetic theory.

Part 4 of 5Solutions, solubility and curves

TermMeaning
SoluteSubstance that dissolves
SolventLiquid in which it dissolves
SolutionSolute and solvent mixture
Saturated solutionHolds the maximum dissolved solute
Soluble / insolubleWill / will not dissolve

Solubility is the mass dissolved in a given mass of solvent, often g per 100 g of water.

1
For most solids it increases with temperature (sodium chloride changes little); for gases solubility rises with pressure and falls with temperature.
2
On a solubility curve, to find the mass in a different amount of water scale proportionally: if 68 g of potassium nitrate dissolves in 100 g at 50°C, then 20 g of water dissolves 68 x 20/100 = 13.6 g.
3
For crystals formed on cooling, subtract the two solubilities and scale: 118 - 64 = 54 g per 100 cm3, so 108 g from 200 cm3.
4
In the practical, ammonium chloride is dissolved in measured volumes of water and the temperature at which crystals first appear on cooling is recorded; solubility = mass / volume x 100.

Typical exam use

  • describe how the particles move in each state, use an increase in temperature to explain faster diffusion, and read a solubility curve to find the mass of crystals that appear on cooling.
  • Practise giving the correct term: "melting" not "dissolving", and "evaporation" not "boiling" when the change happens at the surface below the boiling point.

Part 5 of 5Worked examples

  1. Explain why gases diffuse faster than liquids. [2]2 marks

  2. Give two differences between evaporation and boiling. [2]2 marks

  3. Using the data in the notes: how much potassium nitrate dissolves in 20 g of water at 50°C if 68 g dissolves in 100 g? [1]1 mark

  4. Name the change from solid iodine directly to a gas. [1]1 mark

Key terms

TermMeaning
Melting pointTemperature at which a solid melts.
EvaporationChange from liquid to gas at the surface below the boiling point.
SublimationSolid to gas directly.
Saturated solutionSolution holding the maximum solute.
Solubility curveGraph of solubility against temperature.
DiffusionSpreading of particles by random motion.

Avoid these mistakes

1
Saying particles expand when heated — they move further apart.
2
Confusing evaporation with boiling.
3
Stating melting breaks covalent bonds in molecules.
4
Plotting solubility against volume rather than temperature.

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