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1.1.3Additional lessons

Changes of State — IB Chemistry HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 2 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 4Names of the Changes

1.1.3 Changes of State

Each change of state involves a change in the arrangement and energy of the particles.

Melting
  • Solid to liquid, endothermic, as particles gain enough energy to leave fixed lattice positions.
Freezing
  • Liquid to solid, exothermic, as particles lose energy and settle into fixed positions.
Vaporisation
  • Liquid to gas, endothermic, as particles gain enough energy to overcome the forces between them.
Condensation
  • Gas to liquid, exothermic, as particles slow down and attractive forces pull them close together.

Part 2 of 4Sublimation and Deposition

Sublimation and Deposition
Sublimation
  • Solid goes directly to gas without becoming liquid, as shown by iodine and solid carbon dioxide at normal pressure.
Deposition
  • Gas goes directly to solid, as when water vapour forms frost on a cold surface.
Evaporation
  • Occurs below the boiling point at the surface of a liquid, when the fastest particles escape.
Boiling
  • Occurs at the boiling point throughout the liquid, where bubbles of vapour form inside it.

Part 3 of 4Heating Curve

When a solid is heated steadily the temperature changes in stages.

1
In the solid region the temperature rises because the energy supplied increases the average kinetic energy of the vibrating particles.
2
At the melting point the temperature stays constant as the energy supplied overcomes attractions between particles.
3
In the liquid region the temperature rises again as the average kinetic energy increases.
4
At the boiling point the temperature stays constant again while the liquid becomes a gas.
5
In the gas region the temperature rises further as the gas particles gain kinetic energy.

Part 4 of 4Exam Checklist & Tips

1.1.3 – Key Points for the Exam

  • Melting point The temperature at which a pure solid changes to a liquid.
  • Sublimation The change of state from solid directly to gas.
  • Endothermic A change that takes in energy from the surroundings.
  • Exothermic A change that releases energy to the surroundings.

Common Mistakes

  • Stating that the temperature rises during a change of state; it stays constant.
  • Saying that energy goes into increasing kinetic energy at the melting point, when it goes into overcoming attractions.
  • Confusing evaporation, which is at the surface, with boiling, which is throughout the liquid.
  • Thinking that bonds within molecules break when a molecular substance boils.

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