Molecular Matter (Cambridge (CIE) IGCSE Physics): Revision Note

Arrangement & motion of particles

• All particles are in constant motion at any temperature

  • The motion and arrangement of particles are different for each state of matter

• In a solid:

  • The particles are very close together and arranged in a regular pattern

  • The particles vibrate about fixed positions

• In a liquid:

  • The particles are still close together (no gaps) but are no longer arranged in a regular pattern

  • The particles are able to slide past each other

• In a gas:

  • The particles are widely separated, about 10 times further apart in each direction

  • The particles move about randomly at high speeds

The forces & distances between particles

Extended Tier Only

Intermolecular forces and motion of particles

• The forces between particles affect the state of matter

• This is because the magnitude of the forces affects the relative distances and motion of the particles

• This affects the ability of the substance to

  • Change shape

  • Change volume

  • Flow

• The word particles can refer to:

  • Atoms

  • Molecules

  • Ions

  • Electrons

Solids

• The particles in a solid are held in place by strong intermolecular forces

  • They only vibrate in position

  • The distance between them is fixed and is very small

  • This gives the solid its rigid shape and fixed volume

Liquids

• The particles in a liquid have enough energy to move past one another, but not enough to escape the forces holding them together

• They are still held close together

  • The liquid has a fixed volume, similar to that of the solid

• Particles can move around (by sliding past each other)

  • This allows the liquid to change shape and flow

• Liquids are difficult to compress because particles are close together and the repulsive forces between them are large

Gases

• The particles in a gas have more energy and move randomly at high speeds

  • The particles have overcome the forces holding them close together

• Because of the large spaces between the molecules

  • The gas can easily be compressed and is also able to expand

  • Gases flow freely

Temperature & energy of particles

• As the temperature of a gas increases, so does the average speed of particles in the gas

  • At higher temperatures, the particles have more kinetic energy

• Therefore, there must be a temperature at which the particles have the least kinetic energy

• The temperature at which particles have the least kinetic energy is called absolute zero

  • This is the lowest possible temperature, and absolute zero has a value of −273 °C