2.2.3Thermal Physics
Evaporation — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 9 of 16
Part 1 of 6What evaporation is
What evaporation is
Evaporation
- Evaporation change of a liquid to a gas at any temperature, only from the surface of the liquid
Particle energies
- Particle energies molecules in a liquid have a range of energies and the average relates to temperature
Escape
- Escape the more energetic molecules near the surface overcome the intermolecular forces and leave
Part 2 of 6Why evaporation cools
Energy loss
- Energy loss the particles with the most energy leave, so the average energy of the liquid falls
Result
- Result the temperature of the remaining liquid falls
Contact
- Contact an object touching the evaporating liquid transfers thermal energy to the cooler liquid and cools down
Uses
- Uses some refrigerators and air conditioning units, and sweating
Part 3 of 6Factors affecting the rate
Temperature
- Temperature higher temperature gives molecules more kinetic energy, so more can escape
- Higher temperature gives a faster rate
Surface area
- Surface area molecules escape only at the surface
- Larger area gives a faster rate
Air movement
- Air movement wind or a fan carries vapour away and replaces it with drier air
- More air movement gives a faster rate
Volume
- Volume rate depends on surface area, not on the volume of liquid
Part 4 of 6Boiling and evaporation compared
| Feature | Boiling | Evaporation |
|---|---|---|
| Change of state | Liquid to gas | Liquid to gas |
| Temperature | Boiling point only | Any temperature between melting and boiling point |
| Where | Throughout the liquid, with bubbles | From the surface only |
Part 5 of 6Condensation and closed containers
Condensation
- Condensation vapour particles lose energy, for example on a cold surface, and rejoin the liquid
- Dew and mist on cold windows are examples
Closed container
- Closed container evaporation and condensation reach a balance and the air becomes saturated
- No net loss of liquid although particles still move both ways
Open air
- Open air wind and ventilation keep removing vapour, so net evaporation continues
Drying
- Drying washing dries best when warm, windy, dry and spread out with a large surface area
Part 6 of 6More from the specification
1Latent heat of vaporization in joules
- Latent heat of vaporization in joules energetic surface liquid molecules absorb thousands of joules per gram to escape intermolecular cohesion.
2Cooling Effect of Evaporation
- The most energetic liquid molecules break intermolecular bonds and escape into the vapor phase,
- which decreases the average kinetic energy of the remaining substance and cools the system.
3Specific Heat Capacity
- The amount of heat energy required to raise the temperature of 1 kg of a substance by 1 degree Celsius;
- liquid water has an exceptionally high specific heat capacity of 4200 J/(kg*C).
4Thermal Expansion of Matter
- Supplying thermal energy increases particle vibrations and separations in a solid or liquid substance, leading to volumetric expansion.
5Determining Heat Capacity Electrically
- In an experimental system, an electric immersion heater supplying a steady current and voltage allows thermal energy input to be measured via E = IVt to determine specific heat capacity.
6Potential energy
- Energy stored by the configuration of a system.
7Potential difference
- Work done by a unit charge passing through a component.
8Capacity
- Charge a cell can supply, in A h or C.
9Expansion
- Increase in size on heating as particles move further apart.
10Current
- Flow of electrons in a circuit.
11Heater
- A device that transfers electrical energy to the thermal store.
12Thermometer
- A device that reaches equilibrium with an object to measure its temperature
13Joule
- Unit of energy, 1 kg·m2/s2.
14Constant
- Not changing.
15Immersion heater
- Electric heater placed in a liquid or block to supply energy
16ΔE
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
| Term | Meaning |
|---|---|
| Evaporation | Escape of energetic molecules from a liquid surface at any temperature |
| Boiling | Change to gas throughout a liquid at its boiling point |
| Condensation | Vapour becoming liquid as it loses energy |
| Saturated vapour | Air holding as much vapour as it can, with escape and return rates equal |
Avoid these mistakes
1
Saying evaporation happens only at the boiling point — it occurs at any temperature below it.
2
Saying all the particles escape — only the most energetic surface particles do.
3
Saying rate depends on volume — it depends on surface area.
4
Giving two differences from boiling that are really the same, such as surface only and no bubbles.
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