1.1.3Additional lessons
Changes of State — IB Chemistry SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 2 of 98
Part 1 of 4The Changes
State changes alter the arrangement and energy of particles but not the identity of the substance.
Melting and freezing
- Solid to liquid and liquid to solid;
- melting needs energy to loosen the particles from their fixed lattice positions.
Boiling
- Liquid to gas throughout the bulk of the liquid when vapour pressure equals the external pressure.
Evaporation
- Liquid to gas at the surface only, at any temperature, as the most energetic particles escape.
Condensation
- Gas to liquid as particles lose kinetic energy and the attractive forces draw them together.
Sublimation and deposition
- Solid directly to gas, as for solid CO₂ or iodine, and the reverse change gas directly to solid.
Part 2 of 4Reading a Heating Curve
Heating a solid steadily gives a graph of temperature against time with flat sections.
1
In the solid region the temperature rises as the particles vibrate more strongly about their fixed positions.
2
At the melting point the line is flat, because energy supplied is used to overcome intermolecular forces and not to raise temperature.
3
In the liquid region the temperature rises again until the boiling point is reached.
4
At the boiling point the line is flat once more, and the longer flat section shows that boiling needs more energy than melting.
5
Once all the liquid has become gas, further heating raises the temperature of the gas.
Part 3 of 4Energy Changes
Energy Changes
Endothermic changes
- Melting, boiling and sublimation absorb energy from the surroundings to separate the particles.
Exothermic changes
- Freezing, condensation and deposition release energy as attractive forces between the particles reform.
Potential energy
- During a flat section the potential energy of the particles rises while their average kinetic energy stays constant.
Pure substances
- A pure substance melts at one sharp temperature, but an impure one melts over a range at a lower temperature.
Part 4 of 4Exam Checklist & Tips
1.1.3 – Key Points for the Exam
- Sublimation The direct change from solid to gas without passing through the liquid state.
- Melting point The temperature at which a pure solid changes to a liquid at a given pressure.
- Boiling point The temperature at which a liquid's vapour pressure equals the external pressure.
- Evaporation Surface vaporisation of a liquid that can occur at any temperature.
Common Mistakes
- Saying that the temperature rises during melting;
- it stays constant while energy goes into breaking attractions.
- Claiming that bubbles in boiling water are hydrogen and oxygen; they are water vapour.
- Confusing boiling, which occurs throughout the liquid, with evaporation, which occurs at the surface.
- Believing that particles themselves melt or expand when a substance changes state.
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