Theme B — The Particulate Nature of Matter
B.1.7 Changes of State — IB Physics HL Revision Notes & Topical Questions
5 min read 9 parts Lesson 9 of 53 1 quiz
Lesson summary
B.1.7 Changes of State — IB PHYSICS SL notes: Describe melting, boiling, evaporation, condensation and freezing in terms of particles and interpret heating curves.Phase change — matter changes from one phase (solid, liquid or gas) into another without a change in chemical composi…
- Energy and temperature
- Fixed temperatures
- Sloping sections
- Flat sections
- Cooling curve
- Plateau length
Part 1 of 7Changes of state
1Phase change
- Phase change matter changes from one phase into another without a change in chemical composition.
- (solid, liquid or gas)
2Energy and temperature
- Energy and temperature the transfer of thermal energy does not change the temperature during the change.
- The kinetic energy of the molecules is unaffected.
- Only the potential energy (the spacing of the particles) changes.
| Change | From to | Energy |
|---|---|---|
| Melting | Solid to liquid | Absorbed |
| Freezing | Liquid to solid | Released |
| Vaporisation (boiling) | Liquid to gas | Absorbed |
| Condensation | Gas to liquid | Released |
| Sublimation | Solid to gas | Absorbed |
| Deposition | Gas to solid | Released |
Fixed temperatures
- Fixed temperatures melting and freezing occur at the melting or freezing point,
- and vaporisation and condensation at the boiling point at a given pressure.
Water
- Water the freezing point is 0 °C and the boiling point is 100 °C.
Part 2 of 7Heating curve
Heating curve
- Heating curve it shows how the temperature of a substance changes with time or energy.
Sloping sections
- Sloping sections the temperature changes, and the energy changes the average kinetic energy of the particles.
Flat sections
- Flat sections the temperature is constant during a phase change, and the energy changes only the potential energy.
1
Solid heating — the temperature rises as the vibrations increase.
2
Melting plateau — the energy breaks the rigid arrangement and raises potential energy.
3
Liquid heating — the temperature rises again.
4
Boiling plateau — the energy separates the particles completely, a much larger potential energy increase, so the plateau is longer.
5
Gas heating — the temperature rises with a steep gradient as the gas has low heat capacity.
- Cooling curve
- the mirror image, with plateaus at the condensation and freezing points as energy is released.
Part 3 of 7Reading curves
Axes
- Axes temperature is on the vertical axis and time or energy on the horizontal axis, with constant power assumed.
Slope
- ΔT/Δt = P/(mc), so a steeper segment means a smaller specific heat capacity.
Plateau length
- Plateau length it is mL/P, so a longer plateau means a larger latent heat.
Liquid water
- Liquid water its specific heat capacity is about twice that of ice, so the slope is shallower.
Energy on the x-axis
- Energy on the x-axis the gradient gives 1/(mc) and the plateau width gives mL.
Part 4 of 7Evaporation compared with boiling
| Feature | Evaporation | Boiling |
|---|---|---|
| Where | Surface only | Throughout the liquid |
| Temperature | Any temperature | Fixed at a given pressure |
| Speed | Relatively slow | Rapid, with bubbles |
| Cooling | Remaining liquid cools | Needs continued energy supply |
Evaporation mechanism
- Evaporation mechanism the fastest molecules near the surface, from the high-energy tail, escape.
- The average kinetic energy of the remaining liquid falls, so it cools.
Rate of evaporation
- Rate of evaporation it increases with temperature, surface area, a draught and lower humidity.
Boiling mechanism
- Boiling mechanism it occurs when the saturated vapour pressure equals the external pressure.
Pressure
- Pressure the boiling point falls at altitude and rises in a pressure cooker.
Part 5 of 7Energy and the particle picture
Energy and the particle picture
Latent heat
- Latent heat the energy for a change of state at fixed temperature is Q = mL.
No kinetic energy change
- No kinetic energy change the mean kinetic energy stays the same, so the temperature stays constant.
Potential energy
- Potential energy it increases as the particles move apart against the attractive forces.
Boiling
- Boiling extra work is done pushing back the atmosphere to make room for the vapour.
Condensation or freezing
- Condensation or freezing the energy is released to the surroundings, which is why steam burns are severe.
Part 6 of 7Volume changes
Melting
- Melting most substances expand; ice is an exception, expanding on freezing.
Boiling
- Boiling water increases in volume by roughly 1600 times when it becomes steam at atmospheric pressure.
Link
- Link the changes relate to density and the spacing of the particles.
Part 7 of 7More from the specification
1
Latent heat of fusion in melting solids
- Latent heat of fusion in melting solids adding thermal energy overcomes intermolecular bonds, transforming crystalline solids into mobile liquid phases.
2
Thermal Energy Transfer Mechanisms
- Thermal energy is transferred via conduction through vibrating particles and free electrons, convection through bulk fluid circulation,
- (as in copper metal)
- and thermal radiation through electromagnetic waves that propagate through empty space to Earth.
3
Stefan-Boltzmann Law and Stellar Luminosity
- The total power or luminosity radiated by a black-body object of surface area A and absolute temperature T is given by the Stefan-Boltzmann equation P = sigma * A * T4,
- where sigma is the Stefan-Boltzmann constant.
4
Wien's Displacement Law
- The peak emission wavelength lambda_max of thermal radiation emitted by a hot body is inversely proportional to its thermodynamic temperature: lambda_max * T = 2.898 x 10-3 m K, explaining why hotter objects emit light shifted toward shorter wavelengths.
5
Latent Heat and Changes of Phase
- During a change of state, supplying thermal energy breaks intermolecular bonds without altering temperature, governed by Q = m * L.
- (such as fusion or vaporization)
6
Intensity
- Power per unit area perpendicular to the wave direction.
7
Apparent brightness
- The power received per unit area at the observer
8
Thermal equilibrium
- Constant temperature because energy is absorbed and lost at equal rates
9
Thermal conductivity
- Material constant relating the rate of conduction to the temperature gradient and area
10
Radiation
- Transfer by electromagnetic waves, requiring no medium
11
Copper
- Metal widely used for electrical wires.
12
Convection
- Transfer of thermal energy by the movement of a heated fluid
13
Conduction
- Transfer of thermal energy by lattice vibrations and free electrons
14
Luminosity
- The total power of radiation emitted by a star.
15
Wavelength
- Distance between a point on one wave and the same point on the next wave
16
Fusion
- Joining of light nuclei into a heavier one.
Key terms
| Term | Meaning |
|---|---|
| Phase change | Change of matter from one state to another |
| Melting point | Temperature at which a solid becomes a liquid |
| Boiling point | Temperature at which a liquid boils at a given pressure |
| Evaporation | Escape of fast molecules from the surface of a liquid at any temperature |
| Heating curve | Graph of temperature against time or energy for a substance being heated |
| Sublimation | Change from solid directly to gas |
Avoid these mistakes
1
Saying temperature rises during a change of state — it stays constant.
2
Saying energy supplied on a plateau is lost — it raises the potential energy.
3
Confusing evaporation with boiling — evaporation happens at the surface at any temperature.
4
Saying the boiling point is fixed for all pressures — it depends on the external pressure.
5
Saying evaporation warms the liquid — the fastest molecules leave, so it cools.
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