Edexcel IGCSE Physics · Spec 5.6-5.9
Particle Model & Change of State
The particle model of matter and the changes of state.
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Particle Model & Change of State
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Explained
The particle model, and what happens at a change of state
Everything is made of particles, and the three states differ in how those particles are arranged, how they move, and how much energy they have.
In a solid the particles are packed closely in a fixed regular arrangement and can only vibrate about their positions, so a solid keeps its shape and volume.
In a liquid the particles are still close together but randomly arranged and able to slide past each other, so a liquid flows and takes the shape of its container while keeping a fixed volume.
In a gas the particles are far apart and move rapidly in all directions, so a gas fills its container and can be compressed.
Heating and the changes of state
Heating gives the particles more energy. Melting, evaporating and boiling all go towards the gas state and require energy to be supplied. Condensing and freezing go the other way and release energy to the surroundings.
Sublimation is solid straight to gas without a liquid stage.
All of these are physical changes. The substance is chemically identical afterwards, and the change can be reversed, which is why melting ice and freezing water give you the same substance back.
Why the temperature pauses
During a change of state the temperature stays constant even though energy is still being supplied.
That energy is being used to overcome the forces of attraction between the particles, separating them enough to slide past one another or to move apart entirely. It is not increasing their kinetic energy, and since temperature depends on average kinetic energy, the temperature does not rise.
Once the change of state is complete, the energy goes back into increasing kinetic energy and the temperature climbs again. That is why a heating curve has flat sections.
Mass is conserved
Nothing is created or destroyed at a change of state, so the mass stays the same. Ice melting produces exactly the same mass of water.
Density does change, because the volume changes while the mass does not. Water is unusual in being denser than its own solid, which is why ice floats.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks candidates to compare the particles in a solid with those in a gas, for two marks. It credits that the solid particles are in a regular arrangement while the gas particles are randomly arranged, and that the solid particles are closely or tightly packed while the gas particles are widely spaced.
Its notes then rule out three things: ignore statements related to liquids, ignore diagrams, and ignore statements related to movement, including random movement.
Each of those is worth understanding. The question named two states, so material about the third earns nothing however correct it is. A diagram does not substitute for the words when the question asks you to describe. And movement was not what was asked about; arrangement was.
Arrangement means regular or random and close or far apart. Movement means vibrating, sliding past one another, or moving rapidly in all directions. Read which of the two the question wants, then answer that one and nothing else.
Specific heat capacity
Specific heat capacity is the energy needed to raise the temperature of one kilogram of a substance by one degree Celsius, and it explains why different substances warm at different rates for the same energy.
Water's is unusually high, at about 4200 joules per kilogram per degree, which is why it takes a long time to heat and why it is used in central heating systems and as a coolant.
The energy transferred is mass times specific heat capacity times temperature change, and the temperature change is the one quantity here that can be given in either degrees Celsius or kelvin, because a difference of one is the same on both scales.
Spec 5.6-5.9
What you need to know
- Describe the particle model for the three states
- Explain the changes of state
- Understand specific heat capacity
Active recall
Quick check
Answer each question before opening the answer.
Why is a change of state a physical change?
No new substance is made and it can be reversed — only the arrangement and energy of the particles changes.
During melting or boiling, where does the energy go if the temperature stays constant?
Into breaking the bonds or forces between particles, rather than raising the temperature.
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