Particle Theory and its Limitations

Section 1 of 7The three states (HT only)

State	Arrangement	Movement	Forces between particles
Solid	Regular, close-packed	Vibrate about fixed positions	Strong
Liquid	Close together, irregular	Move around each other	Weaker than in a solid
Gas	Far apart, random	Move rapidly in all directions	Almost none

Section 2 of 7Heating a solid (HT only)

1Energy absorbed

• Energy absorbed Thermal energy is transferred to the particles and becomes kinetic energy.

2Vibrations increase

• Vibrations increase The particles vibrate more strongly, and the solid expands slightly.

3Melting

• Melting At the melting point the particles have enough energy to overcome the forces holding them in position, so the solid becomes a liquid.

4Evaporation and boiling

• Evaporation and boiling Some surface particles escape as the liquid is heated.
  • At the boiling point all the particles have enough energy to break free from one another and the liquid boils.

Section 3 of 7Strength of forces (HT only)

Stronger forces

• Stronger forces The stronger the forces between particles, the more energy is needed to melt or boil the substance,
  • so melting and boiling points are higher.

Temperature stays constant

• Temperature stays constant On a heating curve the flat sections show a change of state.
  • The energy is used to break forces between particles, not to raise the temperature.

Energy and state

• Energy and state Changing state is a physical change. The particles themselves are not changed.

Section 4 of 7Predicting the state

1

Find the melting point and boiling point of the substance in the data given.

2

If the temperature is below the melting point, the substance is a solid.

3

If the temperature is between the melting point and boiling point, it is a liquid.

4

If the temperature is above the boiling point, it is a gas.

5

Example: a substance melting at −215.6 °C and boiling at −173 °C is a gas at −150 °C because that is above the boiling point.

Section 5 of 7Limitations of the model (HT only)

Particles are all the same

• Particles are all the same The model treats every particle as a small, solid, inelastic sphere, whatever the substance.

Types of particle ignored

• Types of particle ignored It does not distinguish atoms, ions and molecules, or mixtures of them.

Forces ignored

• Forces ignored It does not show the size of the forces between particles, which differ between substances.

Section 6 of 7Key terms

Term	Meaning
Particle theory	A model that explains the properties of solids, liquids and gases using the arrangement, movement and energy of particles.
Melting point	The temperature at which a solid changes to a liquid.
Boiling point	The temperature at which a liquid changes to a gas throughout the liquid.
Evaporation	The escape of particles from the surface of a liquid at temperatures below its boiling point.
Intermolecular force	An attraction between separate molecules.
Heating curve	A graph of temperature against time as a substance is heated.

Section 7 of 7Exam technique

1

Saying particles themselves melt or expand. They stay the same size; the spacing and energy change.

2

Writing that the bonds inside molecules break when a simple molecular substance melts. Only the weak forces between molecules are overcome.

3

Forgetting that the flat part of a heating curve is a change of state, not a pause in heating.

4

Mixing up the orderbelow melting point is solid, between is liquid, above boiling point is gas.

5

Giving limitations without detail. State that the model shows particles as identical solid spheres and ignores forces between them.