Molecular Matter (Cambridge International IGCSE Science)

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

Forces & distances between particles

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

Worked Example Two states of matter are described below. Identify each of the states of matter.

Substance 1

• molecules are spaced very far apart
• molecules move very quickly at random
• molecules move in a straight line

Substance 2

• molecules are quite closely packed together
• molecules move about at random
• molecules do not have fixed positions

Answer:

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