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

Arrangement & motion of particles

• All molecules and matter are in motion at room temperature

  • The motion and arrangement of particles must be known for each state of matter

• In a solid:

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

  • The molecules vibrate about fixed positions

• In a liquid:

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

  • The molecules are able to slide past each other

• In a gas:

  • The molecules are widely separated - about 10 times further apart in each direction

  • The molecules move about randomly at high speeds

Properties of states of matter

The forces & distances between molecules

Extended tier only

Intermolecular forces and motion of particles

• The forces between molecules and matter (or '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 molecules 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 molecules in a liquid have enough energy to overcome the forces between them

• They are still held close together

  • The volume of the liquid is the same as the volume of the solid

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

  • This allows the liquid to change shape and flow

Gases

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

  • The molecules 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

• The amount of pressure that a gas exerts on its container is dependent on the temperature of the gas

  • This is because particles gain kinetic energy as their temperature increases

• Therefore, there must be a temperature at which the particles are stationary

  • This is the lowest possible temperature, as particles cannot travel any slower than 0 m/s

• The temperature at which all particles are stationary is called absolute zero

  • Absolute zero has a value of −273 °C