Kinetic Particle
Theory

IP Chemistry (6092), Sec 3

By the end of this topic, you can answer

1

Why can you pour water but not a rock, and why can you squash air but not water?

2

What happens to the particles when ice melts?

3

Why does the temperature stay the same while water boils, even though you keep heating it?

4

What is the difference between evaporation and boiling?

5

How can you smell cooking from across the room?

6

Why does a gas diffuse faster when it is hot, or when its particles are light?

Starter

PredictSomeone opens a bottle of perfume at the front of the classroom. You are sitting at the back.

Why do you smell it after a short while, even though nobody walks over to you?

NoteThe answer comes from the kinetic particle theory. It explains solids, liquids and gases, and how they change.
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.
1

Solids, Liquids and Gases

1.1Picture the particles

  • The particles may be atoms, molecules or ions.
  • How the particles are arranged and how they move decide whether a substance is a solid, a liquid or a gas.
SolidPacked closely in a regular pattern. Each particle vibrates about a fixed position.
LiquidClosely packed but in no regular pattern. Particles slide past one another.
GasFar apart, in no pattern. Particles move fast and at random in every direction.
1

Solids, Liquids and Gases

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
NoteA liquid takes the shape of its container and a gas fills its container.
1

Solids, Liquids and Gases

Quick checkWhich statement about the particles in a gas is correct?

AThey are packed in an orderly pattern.
BThey vibrate about fixed positions.
CThey are far apart and move quickly in all directions.
DThey are close together and slide past one another.
NoteC: gas particles are far apart with weak forces of attraction. A is a solid, B is a solid and D is a liquid.
1

Solids, Liquids and Gases

1.3Changes of state

Red arrows absorb energy. Blue arrows release energy.

1

Solids, Liquids and Gases

1.3Energy in and energy out

Energy absorbedThe particles gain energy: melting, boiling (and evaporation) and sublimation.
Energy releasedThe 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 the reverse, e.g. iodine vapour forming crystals on a cold surface.
1

Solids, Liquids and Gases

1.4Heating curve

  1. A to B, C to D and E to F: 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 overcomes the forces of attraction and increases the potential energy as the particles move apart. Kinetic energy does not change, so the temperature stays constant.
1

Solids, Liquids and Gases

PredictYou keep heating ice at its melting point with a steady flame.

Why does the temperature not rise while the ice is melting?

NoteThe energy is used to overcome the forces of attraction between the particles, raising their potential energy. It is not used to make the particles move faster.
AnalogyPulling two magnets apart takes energy, but they do not speed up. The energy is stored in their new positions. That stored energy is potential energy.
NoteWhile melting, two states are present together: the solid and the liquid.
1

Solids, Liquids and Gases

1.5Cooling curve

  1. A to B, C to D and E to F: 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

Solids, Liquids and Gases

Quick checkA liquid is cooled steadily and its temperature stays at 37 °C for several minutes. What is happening?

AThe liquid is evaporating.
BThe liquid is freezing and releasing energy.
CThe solid is melting and absorbing energy.
DThe liquid is boiling.
NoteB: during cooling, a flat section means a change of state. Energy is released as the particles are pulled closer together, which keeps the temperature the same.
1

Solids, Liquids and Gases

1.6Expansion and contraction

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

Solids, Liquids and Gases

1.7Evaporation and boiling

EvaporationBoiling
Temperatureany temperatureonly at the boiling point
Whereat the surface onlythroughout the liquid, with bubbles
Rateslowfast
NoteFaster particles at the surface of a liquid escape into the air. This is why wet clothes dry on a warm day.
2

How Particles Move

2.1Diffusion

Diffusion is the net movement of particles from a higher concentration to a lower concentration.
Ink spreading through water (speeded up)
Bromine spreading into air (speeded up)
2

How Particles Move

2.1Diffusion around us

  • In liquids: sugar sweetens tea throughout without stirring, and ink spreads through water.
  • In gases: brown bromine gas spreads up into the jar of air above it, and perfume spreads through a room.
  • The particles keep moving once evenly spread, but there is no net change in concentration.
AnalogyWhen a packed train stops at an empty platform, people spill out and spread along the platform, even though nobody tells them where to go. Random movement alone carries them from crowded to empty.
2

How Particles Move

2.2What affects the rate of diffusion

TemperatureA higher temperature gives particles more kinetic energy, so the rate of diffusion increases.
Particle massHeavier particles move more slowly, so the rate of diffusion decreases.
2

How Particles Move

2.2The white ring

  • 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.
2

How Particles Move

Quick checkAmmonia gas and hydrogen chloride gas are released at opposite ends of a long tube. A white ring forms. Where does it form?

AIn the middle of the tube.
BNearer the hydrogen chloride end.
CNearer the ammonia end.
DAt the hydrogen chloride end itself.
NoteB: ammonia particles are lighter, so they diffuse faster and travel further before meeting the hydrogen chloride particles.
2

How Particles Move

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

Step 1Heating gives some particles at the surface enough kinetic energy to escape into the air as a vapour.

Step 2The particles are in constant random motion, so they diffuse through the air from the food to your nose.

Exit ticket

1Name the change of state when a solid turns straight into a gas.

Sublimation.[1]

2Why is a gas easy to compress but a liquid is not?

Gas particles are very far apart, while liquid particles are already closely packed.[1]

3Give one difference between evaporation and boiling.

Evaporation happens at any temperature, but boiling happens only at the boiling point.[1]

4Which diffuses faster at the same temperature, ammonia or hydrogen chloride?

Ammonia, because its particles are lighter.[1]