IGCSE Chemistry 0620 — Topic 1
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States of Matter

Solids, Liquids, Gases & Phase Changes

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Learning Objectives

Explain the three states of matter using particle theory

Describe the properties of solids, liquids, and gases

Understand physical changes and phase transitions

Interpret heating and cooling curves

Explain diffusion in gases and liquids

The Particle Model

All substances are composed of tiny particles (atoms, molecules, ions) that are constantly moving.

Key principles:
• Particles have mass and occupy space
• Particles are in constant, random motion
• Particles attract each other (intermolecular forces)
• Temperature affects particle motion speed

Solids

Structure

  • Particles tightly packed
  • Arranged in fixed pattern (crystalline)
  • Strong forces between particles
  • Particles vibrate in fixed positions

Properties

  • Fixed shape
  • Fixed volume
  • Incompressible
  • High density

Liquids

Structure

  • Particles close together
  • Random arrangement
  • Moderate forces between particles
  • Particles move freely

Properties

  • No fixed shape (takes container shape)
  • Fixed volume
  • Slightly compressible
  • Medium density

Gases

Structure

  • Particles far apart
  • Random arrangement
  • Weak forces between particles
  • Particles move rapidly

Properties

  • No fixed shape (fills container)
  • No fixed volume
  • Highly compressible
  • Very low density

Physical Changes

A physical change does NOT produce a new substance. The particles remain the same.

Phase changes:
• Melting: Solid → Liquid (heat absorbed)
• Freezing: Liquid → Solid (heat released)
• Evaporation: Liquid → Gas (heat absorbed)
• Condensation: Gas → Liquid (heat released)
• Sublimation: Solid → Gas (heat absorbed)
• Deposition: Gas → Solid (heat released)

Energy in Phase Changes

Melting: Break some forces; particles gain kinetic energy

Evaporation: Break most forces; particles escape as gas

Sublimation: Direct solid → gas transition (e.g., dry ice)

Heating Curves

A heating curve shows temperature vs. time as a substance is heated.

Key features:
• Sloped sections: Temperature increases (particles move faster)
• Flat sections (plateaus): Phase change occurring
• Melting plateau: Temperature constant while melting
• Boiling plateau: Temperature constant while boiling
• Longer plateaus = stronger intermolecular forces

Important: Temperature stays constant during phase changes because energy breaks bonds, not increasing motion.

Cooling Curves

Gas cools (slope down) → Condensation occurs (flat section)

Liquid cools (slope down) → Freezing occurs (flat section)

Solid cools (slope down) → Temperature decreases continuously

Diffusion

Diffusion: Random movement of particles from area of high concentration to low concentration.

Gases diffuse quickly:
• Particles move rapidly
• Large spaces between particles
• No strong forces to restrict movement
• Complete mixing occurs quickly

Example: Perfume spray in a room spreads throughout quickly.

Diffusion in Liquids

Particles are closer together

Stronger intermolecular forces slow movement

Particles collide more frequently

Temperature Effects

Particles move faster (more kinetic energy)

More frequent, energetic collisions

Faster net movement from high to low concentration

Phase Change Summary

Solid ⇌ Liquid ⇌ Gas

Forward (endothermic - requires energy):
Melting | Evaporation | Sublimation

Reverse (exothermic - releases energy):
Freezing | Condensation | Deposition

The same amount of energy released during condensation equals the energy absorbed during evaporation (latent heat of vaporization).

Key Definitions

Melting point: Temperature at which solid becomes liquid

Boiling point: Temperature at which liquid becomes gas

Latent heat: Energy required to change state without temperature change

Intermolecular forces: Attractions between particles in a substance

Diffusion: Random movement causing concentration equalization

Lesson Summary

All matter exists in three states: solid, liquid, gas

Particle model explains state properties and changes

Phase changes require energy (latent heat)

Temperature constant during phase transitions

Diffusion occurs fastest in gases, slowest in solids

Higher temperature increases diffusion rate

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