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2.2.3Thermal Physics

Melting, Boiling AND Evaporation — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 20 of 45 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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2.2.3 Melting, Boiling and Evaporation — IGCSE PHYSICS notes: Describe melting, boiling, condensation, solidification and evaporation in terms of particles and energy.Melting point — pure ice melts at 0 °CBoiling point — pure water boils at 100 °CStandard pressure — these are the…

  • Constant temperature
  • Where the energy goes
  • Arrangement
  • Compared with melting
  • Cooling an object
  • Surface area

Part 1 of 8Fixed points of water

Melting point
  • Melting point pure ice melts at 0 °C
Boiling point
  • Boiling point pure water boils at 100 °C
Standard pressure
  • Standard pressure these are the accepted values at standard atmospheric pressure

Part 2 of 8Melting

Constant temperature
  • Constant temperature while melting, the temperature does not change although energy is still supplied
Where the energy goes
  • Where the energy goes into the potential energy of the molecules, not their kinetic energy
  • It overcomes the intermolecular forces
  • Internal energy rises while temperature stays constant
Arrangement
  • Arrangement ice has a regular lattice with particles vibrating, liquid water is irregular and particles slide over one another

Part 3 of 8Boiling

Boiling
  • Boiling liquid to gas throughout the liquid at the boiling point
Energy
  • Energy transferred to potential energy so the forces between molecules are completely overcome
Temperature
  • Temperature stays constant at the boiling point although energy is supplied
Compared with melting
  • Compared with melting boiling needs the forces to be completely overcome

Part 4 of 8Condensation and solidification

Cooling
  • Cooling energy is transferred away from the system and internal energy falls
Condensation
  • Condensation at the boiling point the loss of potential energy lets particles only flow over one another
  • The gas becomes a liquid
  • Kinetic energy and temperature stay constant
Solidification
  • Solidification at the melting point particles can only vibrate about fixed points
  • The liquid becomes a solid
  • Kinetic energy and temperature stay constant

Part 5 of 8Evaporation

Evaporation
  • Evaporation change from liquid to gas at any temperature and only from the surface
Mechanism
  • Mechanism the most energetic molecules near the surface escape
Cooling
  • Cooling the average energy of the remaining liquid falls, so the liquid cools
Cooling an object
  • Cooling an object an object in contact with the evaporating liquid loses thermal energy to it
  • Used in some refrigerators and air conditioning units

Part 6 of 8Factors affecting evaporation

Temperature
  • Temperature higher temperature gives more energy to the molecules, so evaporation is faster
Surface area
  • Surface area larger area gives more molecules at the surface, so evaporation is faster
Air movement
  • Air movement wind or a fan carries vapour away and brings drier air, so evaporation is faster
Volume
  • Volume the rate depends on surface area, not on volume of liquid

Part 7 of 8Boiling compared with evaporation

FeatureBoilingEvaporation
Change of stateLiquid to gasLiquid to gas
TemperatureOnly at the boiling pointAny temperature between melting and boiling point
WhereThroughout the liquid, with bubblesFrom the surface only

Part 8 of 8More from the specification

1
Latent enthalpy of phase change in joules
  • Latent enthalpy of phase change in joules breaking intermolecular bonds during melting or boiling absorbs significant thermal energy measured in joules without temperature change.
2
Melting and boiling
  • Melting and boiling at the melting or boiling point the temperature stays constant while heat energy breaks the bonds between particles.
3
Evaporation
  • Evaporation fastest particles escape from a liquid surface below boiling point, which cools the remaining liquid.
4
Specific heat capacity
  • Specific heat capacity energy needed to raise the temperature of 1 kg of a substance by 1 degree, so E = m c change in temperature.
5
Immersion heater
  • Immersion heater energy supplied equals power times time, used to measure heating of a substance.
    • (current times voltage times time)
6
Thermal expansion
  • Thermal expansion a heated substance expands because its particles vibrate or move more and take up more space.
7
Melting and boiling
  • Melting and boiling at the melting or boiling point the temperature stays constant while heat energy breaks the bonds between particles.
8
Evaporation
  • Evaporation fastest particles escape from a liquid surface below boiling point, which cools the remaining liquid.
9
Specific heat capacity
  • Specific heat capacity energy needed to raise the temperature of 1 kg of a substance by 1 degree, so E = m c change in temperature.
10
Immersion heater
  • Immersion heater energy supplied equals power times time, used to measure heating of a substance.
    • (current times voltage times time)
11
Thermal expansion
  • Thermal expansion a heated substance expands because its particles vibrate or move more and take up more space.
12
Immersion heater
  • Electric heater placed in a liquid or block to supply energy
13
Potential difference
  • Work done by a unit charge passing through a component.
14
Capacity
  • Charge a cell can supply, in A h or C.
15
Expansion
  • Increase in size on heating as particles move further apart.
16
Current
  • Flow of electrons in a circuit.
17
Heater
  • A device that transfers electrical energy to the thermal store.
18
Thermometer
  • A device that reaches equilibrium with an object to measure its temperature
19
Joule
  • Unit of energy, 1 kg·m2/s2.
20
ΔE
ΔE mcΔθ [1 mark]
What the symbols mean: E, Q or q = energy transferred (J) · m = mass (kg; g in chemistry calorimetry) · c = specific heat capacity (J kg−1 °C−1; water 4.18 J g−1 °C−1) · ΔT or Δθ = temperature change (°C or K)

Key terms

TermMeaning
MeltingSolid becoming liquid at the melting point
BoilingLiquid becoming gas throughout the liquid at the boiling point
CondensationGas becoming liquid as energy is lost
SolidificationLiquid becoming solid, also called freezing
EvaporationEscape of the most energetic molecules from a liquid surface at any temperature

Avoid these mistakes

1
Saying temperature rises during melting or boiling — it stays constant while the energy changes potential energy.
2
Saying the energy disappears — it is stored as potential energy of the particles.
3
Saying evaporation happens only at 100 °C — it happens at any temperature below the boiling point.
4
Describing arrangement only as spacing — contrast the regular solid with the irregular liquid.

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