Molecular Matter (CIE IGCSE Physics)

• 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

  

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• 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 particles that make up matter include
  • Atoms
  • Molecules
  • Ions
  • Electrons

Intermolecular Forces and Motion of Particles

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
  • 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

Substance 1 

Step 1: Identify the distances between the molecules

• • The molecules are spaced far apart
    • This can only describe a gas

Step 2: Identify the motion of the molecules

• • The molecules move quickly, at random and in a straight line
    • This confirms that substance 1 is a gas

Substance 2

Step 1: Identify the distances between the molecules

• • The molecules are closely packed
    • This could describe either a solid or a liquid

Step 2: Identify the motion of the molecules

• • The molecules move at random and do not have fixed positions
    • This confirms that substance 2 is a liquid

• 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

• As the temperature of the gas decreases, the pressure on the container also decreases
• In 1848, Mathematician and Physicist, Lord Kelvin, recognised that there must be a temperature at which the particles in a gas exert no pressure
  • At this temperature they must no longer be moving, and hence not colliding with their container

• This temperature is called absolute zero and is equal to -273 °C

At absolute zero, or -273 °C, particles will have no net movement. It is therefore not possible to have a lower temperature

• The unit kelvin is written as K
  • Note that there is no degree as with Celsius, which is written °C