Particle Model of Matter
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Particle Arrangement in Three States
The three states of matter differ in how particles are arranged and how they move.
Describing particle arrangement and movement is the starting point for almost every particle model question.
Particle Arrangement in Three States — Key Knowledge
- Solid particles vibrate in fixed positions, closely packed, regular pattern
- Liquid particles move around each other, close together but no fixed positions
- Gas particles move quickly in random directions, far apart
Changes of State
Substances change state when heated or cooled. These are physical changes — the material can be recovered.
Students must name each change and link it to the correct direction — heating or cooling.
Changes of State — Key Knowledge
- Melting solid to liquid
- Freezing liquid to solid
- Boiling liquid to gas, at a fixed temperature throughout the liquid
- Evaporation liquid to gas, from the surface at any temperature
- Condensation gas to liquid
- Sublimation solid directly to gas
Conservation of Mass
Mass is conserved when a substance changes state because no particles are added or removed.
A common exam trap — if a gas escapes an open container the mass reading drops, but the particles still exist.
Conservation of Mass — Key Knowledge
- Conservation of mass total mass stays the same during any change of state
- Physical change change of state is reversible — no new substance is formed
Internal Energy
The internal energy of a system is the total kinetic and potential energy of all its particles.
Explains why temperature stays constant during melting or boiling — energy increases potential stores, not kinetic.
Internal Energy — Key Knowledge
- Internal energy sum of kinetic energy and potential energy of all particles
- Heating a substance increases internal energy — particles move faster or bonds weaken
- Change of state energy goes into breaking or forming bonds between particles, not raising temperature
Temperature and Changes of State
When a substance changes state, temperature remains constant even though energy is still being supplied.
The heating curve — with its flat sections at melting and boiling points — is a very common exam diagram.
Temperature and Changes of State — Key Knowledge
- Flat sections on a heating curve temperature constant during melting and boiling
- Energy during state change goes into breaking intermolecular bonds, not increasing kinetic energy
Specific Heat Capacity
The energy needed to raise the temperature of 1 kg of a substance by 1 degrees C.
Delta E = mc Delta theta
change in thermal energy = mass x specific heat capacity x temperature change
Links measurable quantities (mass, temperature change) to the energy transferred — a required practical calculation.
Specific Heat Capacity — Key Knowledge
- Specific heat capacity units J/kg degrees C, varies by material — water has a high value
Specific Latent Heat
The energy needed to change the state of 1 kg of a substance with no change in temperature.
E = mL
thermal energy for a change of state = mass x specific latent heat
Latent heat of vaporisation is always larger than latent heat of fusion for the same substance — more bonds to break.
Specific Latent Heat — Key Knowledge
- Specific latent heat of fusion energy to melt or freeze 1 kg at the melting point
- Specific latent heat of vaporisation energy to boil or condense 1 kg at the boiling point
- Units J/kg
Gas Pressure
Gas pressure is caused by particles colliding with the walls of their container.
Every gas pressure question comes back to collisions — frequency, force, or both.
Gas Pressure — Key Knowledge
- Gas pressure force per unit area from particle collisions with container walls
- More frequent or forceful collisions higher pressure
Temperature and Pressure
Heating a gas in a sealed container increases its pressure.
Explains real-world situations like aerosol cans warning against heating.
Temperature and Pressure — Key Knowledge
- Increasing temperature particles gain kinetic energy, move faster
- Effect on pressure particles hit walls harder and more often — pressure rises
Pressure and Volume
For a fixed mass of gas at constant temperature, increasing the volume decreases the pressure and vice versa.
pV = constant
or p1 V1 = p2 V2
This inverse relationship means halving the volume doubles the pressure — a common calculation question.
Pressure and Volume — Key Knowledge
- Reducing volume particles have less space, hit walls more often, pressure increases
- Pressure-volume relationship pressure x volume = constant, for fixed mass at constant temperature
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Particle Model of Matter
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