Topic 02 · Kinetic Particle Theory

Kinetic Particle Theory

1

Solids, Liquids and Gases

1.1The kinetic particle theory

The kinetic particle theory states that all matter is made up of tiny particles that are in constant, random motion.
solidliquidgas
  • The particles may be atoms, molecules or ions.

1.2Comparing the three states

SolidLiquidGas
Arrangementvery closely packed, in an orderly wayclosely packed, in a disorderly wayvery far apart, in a disorderly way
Movementvibrate and rotate about fixed positionsslide past one anothermove quickly and randomly in any direction
Kinetic energyvery lowlowhigh
Forces of attractionvery strongstrongvery weak
Shape and volumedefinite shape and volumeno definite shape, definite volumeno definite shape or volume
Compressed?nonoyes
8Topic 02 · Kinetic Particle Theory
1

Solids, Liquids and Gases (continued)

1.3Changes of state

SolidLiquidGasmeltingfreezingboilingcondensationsublimationdeposition
Energy absorbed

The particles gain energy: melting, boiling (and evaporation) and sublimation.

Energy released

The particles lose energy: freezing, condensation and deposition.

  • Sublimation is a solid changing straight to a gas, e.g. dry ice (solid carbon dioxide). Deposition is a gas changing straight to a solid.

1.4Heating curve

temperaturetimemelting pointboiling pointABCDEFsolidsolid + liquidmeltingliquidliquid + gasboilinggas
  1. A to B (solid) and C to D (liquid) and E to F (gas): thermal energy is converted to kinetic energy, so the particles move faster and the temperature rises.
  2. B to C (melting) and D to E (boiling): the energy absorbed overcomes the forces of attraction and increases the potential energy of the particles as they move apart. Their kinetic energy does not change, so the temperature stays constant.
NoteIn each flat section of a heating or cooling curve, two states are present together.
Topic 02 · Kinetic Particle Theory9
1

Solids, Liquids and Gases (continued)

1.5Cooling curve

temperaturetimeboiling pointmelting pointABCDEFgasliquid + gascondensationliquidsolid + liquidfreezingsolid
  1. A to B (gas) and C to D (liquid) and E to F (solid): kinetic energy is converted to thermal energy and lost to the surroundings, so the particles slow down and the temperature falls.
  2. B to C (condensation) and D to E (freezing): the forces of attraction pull the particles closer together, so their potential energy falls and energy is released. This keeps the temperature constant.

1.6Expansion and contraction

coldheated
Expansion

Heated particles vibrate more and sit slightly further apart, so a solid expands.

Contraction

Cooled particles lose energy, vibrate less and come closer.

NoteWater is unusual: its particles are more closely packed in liquid water than in ice, so ice is less dense than water and floats.

1.7Evaporation and boiling

EvaporationBoiling
Temperatureany temperatureonly at the boiling point
Whereat the surface onlythroughout the liquid, with bubbles
Rateslowfast
NoteIn evaporation, only the particles at the surface with enough energy escape into the air.
10Topic 02 · Kinetic Particle Theory
2

How Particles Move

2.1Diffusion

Diffusion is the net movement of particles from a higher concentration to a lower concentration.
startlaterevenly spread
Ink spreading through water
startlaterbromineair
Bromine spreading into air
  • Diffusion happens in liquids and gases. It explains everyday effects such as a smell spreading through a room.
  • The particles keep moving once evenly spread, but there is no net change in concentration.

2.2What affects the rate of diffusion

Temperature

A higher temperature gives particles more kinetic energy, so the rate of diffusion increases.

Particle mass

Heavier particles move more slowly, so the rate of diffusion decreases.

white ring of ammonium chloridehydrochloric acidaqueous ammoniacotton wool soaked incotton wool soaked inHClNH₃ (lighter, faster)
  • Ammonia and hydrogen chloride diffuse towards each other and react to form white ammonium chloride.
  • The white ring forms nearer the hydrogen chloride end, because the lighter ammonia particles diffuse faster and travel further.
  • Porous pot: if the gas outside is lighter, it diffuses into the pot faster than the gas inside diffuses out, so the pressure inside rises.

Worked exampleYou can smell cooking food from across a room. Explain this using the kinetic particle theory.

Heating gives some particles at the surface enough kinetic energy to escape into the air as a vapour.The particles are in constant random motion, so they diffuse through the air from the food to your nose.
Topic 02 · Kinetic Particle Theory11