12.1.4Particle Model
Changes of State — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions
5 min read 9 parts Lesson 5 of 13
Part 1 of 7What a change of state is
A change of state is a physical change in which a substance moves between solid, liquid and gas without changing its chemical identity.
What a change of state is
The particles are the same before and after; only their arrangement and energy change.
The substance can return to its original state when the process is reversed, and the mass stays the same because particles are neither created nor destroyed.
Melting
Melting solid to liquid at the melting point.
Freezing
(solidifying): liquid to solid at the same temperature.
Boiling
Boiling liquid to gas throughout the liquid, at the boiling point.
Condensation
Condensation gas to liquid.
Evaporation
Evaporation liquid to gas from the surface only, at any temperature below boiling.
Sublimation
Sublimation solid straight to gas without passing through the liquid state (for example solid carbon dioxide, "dry ice", and iodine).
Part 2 of 7Energy during a change of state
When a solid is heated, its temperature rises until it reaches its melting point.
Then the temperature stays constant while it melts, even though energy is still being supplied.
The energy supplied does not raise the kinetic energy of the particles (which would raise the temperature); it is used to break the bonds holding the particles in fixed positions, increasing their potential energy.
The same thing happens at the boiling point, when a much larger amount of energy is needed to separate the particles completely.
Latent heat
This energy is called latent heat.
When the substance reverses the change (freezing, condensing), the same energy is released to the surroundings, so the temperature stays constant while the substance changes state.
Part 3 of 7Heating and cooling curves
A heating curve plots temperature (y) against time (x) for a substance heated at a steady rate.
Heating and cooling curves
For ice heated from −20 °C the graph has five sections.
- A sloping line as the ice warms to 0 °C.
- A flat line while melting at 0 °C.
- A sloping line as the water warms to 100 °C, less steep because water has a higher specific heat capacity.
- A flat line at 100 °C while boiling, which is longer than the melting plateau because more energy is needed to separate the particles.
- A rising line as the steam heats.
A cooling curve is the reverse: temperature falls, with a plateau at the freezing point.
A pure substance melts and freezes at a single sharp temperature; an impure substance or a mixture melts over a range of temperatures.
Part 4 of 7Evaporation and boiling compared
Boiling occurs throughout the liquid at a specific temperature when bubbles of vapour form inside it.
1
Evaporation happens only at the surface and at any temperature.
2
In evaporation the fastest particles near the surface have enough kinetic energy to escape the attraction of the others.
3
Because the most energetic particles leave, the average kinetic energy of those left behind drops, so the liquid cools; this is how sweating cools you.
4
Evaporation is faster when: the temperature is higher, the surface area is larger, there is a draught or wind removing the vapour, and the air is dry.
Part 5 of 7Latent heat quantity (extension)
The energy needed to change the state of a mass m of substance at constant temperature is Q = mL, where L is the specific latent heat in J kg−1.
Latent heat quantity (extension)
For melting ice, L is about 3.3 × 105 J kg−1; for boiling water, about 2.3 × 106 J kg−1, which is about seven times larger.
The much higher value for vaporisation is why steam can cause more serious burns than boiling water at the same temperature: as it condenses on skin it releases a lot of energy.
Part 6 of 7Applications and worked reasoning
Condensation on a cold window happens when water vapour in warm air touches a cold surface, loses energy and returns to liquid.
Applications and worked reasoning
Fridges and air conditioners depend on evaporation and condensation: a liquid refrigerant evaporates in the cold coils, absorbing energy from the inside of the fridge, then is compressed and condenses outside, releasing the energy to the room.
Sweating cools the body in the same way, as evaporating sweat takes thermal energy from skin.
When designing an experiment on evaporation, change one factor only, such as the surface area, and keep temperature and air flow the same.
When explaining heating curves, link each section to the energy.
1
Sloping sections: energy increases the kinetic energy of particles so temperature rises.
2
Flat sections: energy increases potential energy as bonds break and the state changes.
3
The reverse holds for cooling curves.
4
Mass remains constant throughout a change of state, but density changes: ice is less dense than water, so it floats and pipes may burst when water freezes and expands.
Part 7 of 7Worked examples
Sketch in words the heating curve of ice at −10 °C heated steadily to steam at 110 °C.
Start at −10 °C and rise to 0 °C (ice warming).
1Flat at 0 °C while ice melts.2Rise from 0 °C to 100 °C as the liquid water heats, with a smaller gradient than the first stage because the specific heat capacity of water is about twice that of ice.3Flat at 100 °C while boiling, a longer plateau than melting.4Rise from 100 °C to 110 °C as the steam warms.How much energy is needed to melt 0.40 kg of ice at 0 °C? (L = 3.3 × 105 J kg−1)
Q = mL = 0.40 × 3.3 × 105 = 1.32 × 105 J = 132 kJ.
What the symbols mean: E or Q = energy to change state without a change of temperature (J) · m = mass (kg) · L = specific latent heat (J kg−1)Explain why wet clothes dry faster on a windy day.
Wind carries away water vapour from near the clothes, so fewer particles return to the liquid, and evaporation continues faster. A warm day also gives particles more kinetic energy to escape.
Key terms
| Term | Meaning |
|---|---|
| Melting point | Temperature at which a solid changes into a liquid. |
| Boiling point | Temperature at which a liquid boils at a given pressure. |
| Latent heat | Energy absorbed or released during a change of state without a temperature change. |
| Evaporation | Escape of particles from the surface of a liquid at any temperature. |
| Sublimation | Change from solid directly to gas. |
| Plateau | Flat region on a heating curve where state is changing. |
Avoid these mistakes
1
Saying temperature rises during melting; it stays constant while energy breaks bonds.
2
Thinking boiling and evaporation are the same process.
3
Believing that particles melt or expand themselves.
4
Forgetting that condensing releases energy.
5
Drawing a plateau in a heating curve with a slope.
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