1.3 Particle model of matter

Density definition

Mass per unit volume

Density units

kg/m³ (kilograms per metre cubed)

Why gases have much lower density than solids

Particle spacing in gases is ~10x greater, so volume is much larger and density much smaller

Why solids and liquids have similar densities

The spacing between their particles does not change significantly

Is mass conserved during a change of state?

Yes — mass is always conserved

Are changes of state physical or chemical?

Physical — they are reversible and the material retains its original properties

Melting

Solid → liquid

Freezing

Liquid → solid

Evaporating

Liquid → gas

Condensing

Gas → liquid

Sublimation

Solid → gas directly (e.g. dry ice/solid CO₂)

Internal energy

Energy stored by particles within a system

Two forms of internal energy

Kinetic energy (vibration) and potential energy (between particles)

What does heating a system do?

Increases internal energy

Two effects of increased internal energy

Either raises the temperature OR causes a change of state

Specific Heat Capacity definition

Energy required to raise the temperature of 1 kg of a substance by 1°C

Specific Latent Heat definition

Energy needed to change the state of 1 kg of a substance with no temperature change

Specific Latent Heat formula

E = mL

What does each symbol mean in E = mL?

E = energy (J), m = mass (kg), L = specific latent heat (J/kg)

Specific Latent Heat of fusion

Energy to melt or freeze 1 kg of a substance

Specific Latent Heat of vaporisation

Energy to boil or condense 1 kg of a substance

Is latent heat of fusion or vaporisation greater?

Vaporisation — evaporation requires more energy than melting

Energy absorbed or released during melting/evaporating?

Absorbed

Energy absorbed or released during freezing/condensing?

Released

Why is temperature constant during a change of state?

Energy is used to break particle bonds, not raise temperature

Gas pressure definition

Total force per unit area exerted on container walls by colliding gas molecules

What determines the temperature of a gas?

The average kinetic energy of its molecules

Effect of higher temperature on gas molecules

Greater average kinetic energy → faster average speed

Pressure law

At constant volume, changing temperature changes pressure

Boyle's Law

At constant temperature and fixed mass: P × V = constant

Why does increasing volume decrease pressure (Boyle's)?

Fewer collisions per unit area of the walls

Why does compressing a gas increase pressure?

Particles gain momentum from the inward-moving wall → collide more frequently with walls

Why does compressing a gas increase temperature?

Work done on gas transfers energy to particles → increases kinetic energy → increases temperature

Effect of pumping more gas into a fixed volume

More particles → more collisions per unit time → pressure increases and gas heats up