Molecular Matter (Cambridge International IGCSE Physics)

Arrangement & motion of particles

Spec point — 2.1.2 · Describe the particle structure of solids, liquids and gases in terms of the arrangement, separation and motion of the particles, and represent these states using simple particle diagrams

• 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

Particles are closely packed in solids and liquids, but widely separated in gases

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

Substance 1

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

Substance 2

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

[2]

Answer:

Substance 1

Step 1: Identify the distances between the particles

• The particles are spaced far apart

  • This can only describe a gas

Step 2: Identify the motion of the particles

• The particles move quickly, at random and in a straight line

  • This confirms that substance 1 is a gas [1 mark]

Substance 2

Step 1: Identify the distances between the particles

• The particles are closely packed

  • This could describe either a solid or a liquid

Step 2: Identify the motion of the particles

• The particles move at random and do not have fixed positions

  • This confirms that substance 2 is a liquid [1 mark]

Exam Hint The properties of particles must be known for each state of matter.

In comparison questions, you should name the property and give descriptions that compare the states of matter. Note that particles do not expand, only the average separation changes.

Forces & distances between particles

Spec point — 2.1.2 · Know that the forces and distances between particles (atoms, molecules, ions and electrons) and the motion of the particles affects the properties of solids, liquids and gases

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

Spec point — 2.1.2 · Describe the relationship between the motion of particles and temperature, including the idea that there is a lowest possible temperature (−273°C), known as absolute zero, where the particles have least kinetic energy

• 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