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P2.2.3P2. Thermal Physics

Melting, boiling and evaporation — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 10 parts Lesson 7 of 14 1 quiz

📚 Combined Science 0653 - Physics Hub / Topical Revision
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Lesson summary

P2.2.3 Melting, boiling and evaporation — COMBINED SCIENCE notes: Explain melting, boiling, condensation, solidification and evaporation using the particle model, and compare boiling with evaporation.Melting point — ice melts at 0 °C.Boiling point — pure water boils at 100 °C.Con…

  • Conditions
  • Before melting
  • At the melting point
  • Where the energy goes
  • Before boiling
  • At the boiling point

Section 1 of 10Fixed points of water

Melting point
  • Melting point ice melts at 0 °C.
Boiling point
  • Boiling point pure water boils at 100 °C.
Conditions
  • Conditions these values apply to pure water at standard atmospheric pressure.
Kelvin
  • 0 °C = 273 K and 100 °C = 373 K.

Section 2 of 10Melting

Before melting
  • Before melting the temperature of the solid rises as energy is added.
At the melting point
  • At the melting point the temperature stays constant even though energy is still supplied.
Where the energy goes
  • Where the energy goes it increases the potential energy of the particles by overcoming the forces between them, not their kinetic energy.
Result
  • Result the regular lattice breaks down and the particles slide over one another as a liquid.

Section 3 of 10Boiling

Before boiling
  • Before boiling the temperature of the liquid rises as energy is added.
At the boiling point
  • At the boiling point the temperature stays constant while energy is still supplied.
Where the energy goes
  • Where the energy goes it completely overcomes the forces between the particles, which increases their potential energy and not their kinetic energy.
Result
  • Result the liquid becomes a gas, and boiling occurs at a higher temperature than melting.

Section 4 of 10Condensation and solidification

Condensation and solidification
Condensation
  • Condensation a gas loses energy until it reaches the boiling point, then loses potential energy as particles are pulled together into a liquid.
Solidification (freezing)
  • Solidification (freezing) a liquid loses energy until it reaches the melting point, then loses potential energy as the particles lock into a solid.
Temperature
  • Temperature it stays constant during both changes because the kinetic energy does not change.
Same temperatures
  • Same temperatures condensation happens at the boiling point and solidification at the melting point.
Cooling curve
  • Cooling curve the flat sections show condensation and solidification.

Section 5 of 10Evaporation

Evaporation
Evaporation
Evaporation a change from liquid to gas that happens at any temperature and only at the surface.
Particle explanation
  • Particle explanation the molecules in a liquid have a range of energies, and the most energetic molecules near the surface escape.
Cooling
  • Cooling the average kinetic energy of the remaining molecules falls, so the liquid cools.
Uses
  • Uses sweat cools the skin, and some refrigerators and air conditioners use evaporation.

Section 6 of 10Factors affecting evaporation

Temperature
  • Temperature a higher temperature gives more molecules enough energy to escape, so evaporation is faster.
Surface area
  • Surface area a larger surface area gives more places for molecules to escape.
Air movement
  • Air movement wind or a fan removes vapour and replaces it with drier air, which increases the rate.

Section 7 of 10Boiling compared with evaporation

FeatureBoilingEvaporation
Change of stateLiquid to gasLiquid to gas
TemperatureOnly at the boiling pointAny temperature
Where it happensThroughout the liquid, with bubblesFrom the surface only
Energy supplyContinuous supply neededTakes energy from the liquid itself, which cools

Section 8 of 10More from the specification

1Thermal energy measured in joules
  • Thermal energy measured in joules absorbing thermal heat measured in joules elevates lattice kinetic energy, driving measurable linear expansion.
2Thermal expansion
  • Solids, liquids and gases expand when heated because particles vibrate or move faster and take up more space.
3Specific heat capacity
  • Energy needed to raise 1 kg of a substance by 1 °C; ΔQ = mcΔθ.
4Measuring capacity
  • An electrical heater and a current measurement give the energy transferred, with insulation reducing losses from the system.
5Melting and boiling
  • Energy supplied changes internal potential energy not temperature during a change of state, so the temperature stays constant.
6Evaporation
  • Fastest particles escape from the surface of a liquid, so the remaining liquid cools.
7Thermal expansion
  • Solids, liquids and gases expand when heated because particles vibrate or move faster and take up more space.
8Specific heat capacity
  • Energy needed to raise 1 kg of a substance by 1 °C; ΔQ = mcΔθ.
9Measuring capacity
  • An electrical heater and a current measurement give the energy transferred, with insulation reducing losses from the system.
10Melting and boiling
  • Energy supplied changes internal potential energy not temperature during a change of state, so the temperature stays constant.
11Evaporation
  • Fastest particles escape from the surface of a liquid, so the remaining liquid cools.
12Joule
  • Unit of energy.
13Voltage
  • A measure of the electrical energy supplied or changed in a circuit.
14Internal energy
  • The total kinetic and potential energy of the particles in a system.
15Immersion heater
  • An electric heater placed inside a liquid or block to heat it.
16Intermolecular forces
  • Weak forces between molecules.
17Potential difference
  • The work done by a unit charge passing through a component.
18Expansion
  • The increase in volume of a fluid when heated.
19Current
  • Flow of electrons in a circuit.
20Heater
  • Device that transfers energy thermally.
21Heating
  • Energy transferred from a hotter to a colder object.
22ΔE
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)

Section 9 of 10Key terms

TermMeaning
Melting pointThe temperature at which a solid turns to a liquid, 0 °C for pure water.
Boiling pointThe temperature at which a liquid boils, 100 °C for pure water.
CondensationThe change from gas to liquid.
SolidificationThe change from liquid to solid.
EvaporationThe escape of the most energetic molecules from the surface of a liquid.
Fixed pointA reproducible temperature such as the melting or boiling point of pure water.

Section 10 of 10Exam technique

1
Saying the temperature rises while a substance melts or boils — it stays constant during the change of state.
2
Saying the energy supplied is lost at the melting point — it increases the potential energy of the particles.
3
Saying evaporation only happens at 100 °C — it happens at any temperature.
4
Saying evaporation warms the liquid — the fastest molecules leave, so it cools.

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