Particle model, states and changes of state
Explain states and physical changes using particle arrangement, motion, separation and energy.
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The core idea
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Learning objectives
- describe the solid, liquid and gaseous states of matter and explain their interconversion in terms of the kinetic particle theory and of the energy changes involved.
Your goal is to use particle arrangement, separation, motion and energy to explain states and changes of state without changing the particles themselves.
1. Retrieve what you know
A liquid can flow but is difficult to compress. Before reading on, predict what this tells you about the spacing and movement of its particles.
Check the retrieval
Liquid particles are close together, so there is little empty space to remove, but they are not fixed and can move past one another.
2. Particle model
Solid
Motion: fixed positions; vibrate
Property: fixed shape and volume
Liquid
Motion: move past one another
Property: fixed volume; takes container shape
Gas
Motion: rapid random motion
Property: fills container; compressible
- In a solid, particles are close together in an ordered arrangement and vibrate about fixed positions.
- In a liquid, particles remain close together but are disordered and move past one another.
- In a gas, particles are far apart and move rapidly and randomly.
During melting or boiling, particles absorb energy. During freezing or condensation, particles release energy. State changes alter motion, arrangement or separation; they do not alter particle size or chemical identity.
3. Modelled example: boiling
Modelled example 1
Explain boiling without making new particles
Problem
Explain what happens when liquid water boils.
Study the worked solution
Follow the energy
Method
State that the water absorbs energy.Reason
The added energy increases particle motion and helps particles overcome the attractions keeping them close.
Working
The particles move more rapidly.Describe the gas
Method
State that the particles become much further apart and move rapidly and randomly.
Reason
This describes a gas using the particle model.Working
They remain water particles; boiling is a physical change.
4. Guided attempt: condensation
Guided practice 2
Reverse the energy-and-particle chain
Problem
Water vapour forms liquid drops on a cool surface. Select the energy direction and the particle change.
Try this before viewing the solution
Hints
Hint 1: reverse boiling
Condensation reverses the energy, motion and separation changes in boiling.
Hint 2: keep particle identity
Change average separation, not the size or identity of each particle.
View solution step by step
Reverse boiling
Method
Release energy and reduce average particle speed.
Reason
Condensation is the reverse of vaporisation.Working
Gas particles move less rapidly on average.
Form the liquid
Method
Bring the particles much closer together while allowing them to move past one another.
Reason
Attractions keep liquid particles close without fixing them in place.
Working
Gas becomes liquid; particle identity is unchanged.
5. Misconception contrast
“A substance expands when heated because each particle becomes larger” is incorrect. Heating can increase average separation; each particle keeps its size and identity. Boiling also does not break water particles into hydrogen and oxygen.
6. Changed-context transfer
A solid melts but remains at the same temperature during part of the heating. Explain why energy is still absorbed and describe the arrangement after melting.
Show transfer feedback
Energy is absorbed to overcome attractions sufficiently for particles to leave fixed positions. In the liquid they remain close together but are disordered and move past one another. Constant temperature during the change does not mean that no energy is transferred.
7. Independent check
Without looking back, compare a gas with a liquid in terms of separation and motion, then explain freezing using energy and arrangement. Check that you did not change particle size or identity.