States of Matter

The Three States of Matter

All matter is made of particles (atoms or molecules). The state of matter — solid, liquid, or gas — depends on the amount of energy these particles have and the forces between them.

A fourth state, plasma, exists at extremely high temperatures (e.g., the Sun's interior, lightning) where electrons are stripped from atoms. It is the most common state of matter in the universe.

Properties of Each State

Solid:

• Fixed shape and fixed volume
• Particles are packed tightly in a regular, ordered pattern (crystalline lattice)
• Particles vibrate about fixed positions — they do not move from place to place
• Strong forces of attraction between particles
• Virtually incompressible
• Usually the densest state

Liquid:

• Fixed volume but no fixed shape — takes the shape of its container
• Particles are close together but have enough energy to flow and slide past each other
• Weaker forces than in solids
• Slightly compressible
• Can flow and diffuse

Gas:

• No fixed shape or fixed volume — fills its container completely
• Particles are far apart and move rapidly in random directions
• Very weak forces between particles
• Highly compressible
• Much less dense than solids or liquids
• Particles frequently collide with each other and with container walls

Comparing States of Matter

Property	Solid	Liquid	Gas
Shape	Fixed	Takes container shape	Takes container shape
Volume	Fixed	Fixed	Fills container
Particle spacing	Very close, ordered	Close, disordered	Far apart
Particle movement	Vibrate only	Flow past each other	Move rapidly in all directions
Compressibility	Very low	Very low	High
Density	High	Medium	Very low

Particle Energy and Temperature

Temperature is a measure of the average kinetic energy of particles. As temperature increases:

• Particles move faster and vibrate more
• Forces between particles are overcome
• The substance can change state

Solid → Liquid → Gas (increasing energy)

Phase Changes

A phase change (change of state) occurs when a substance gains or loses enough energy to transition between states. During a phase change, temperature remains constant even though energy is being added or removed — the energy is used to break or form bonds between particles (latent heat).

Change	Name	Energy absorbed or released
Solid → Liquid	Melting	Absorbed
Liquid → Solid	Freezing / Solidification	Released
Liquid → Gas	Evaporation / Boiling	Absorbed
Gas → Liquid	Condensation	Released
Solid → Gas	Sublimation	Absorbed
Gas → Solid	Deposition	Released

Latent heat is the energy required to change the state of a substance without changing its temperature. Every substance has a specific latent heat of fusion (melting) and latent heat of vaporisation (boiling).

Heating and Cooling Curves

A heating curve shows how temperature changes as a substance is heated at a constant rate:

• Temperature rises steadily in each state
• Temperature plateaus (stays flat) during each phase change — the energy goes into breaking bonds, not raising temperature
• After the phase change is complete, temperature rises again

For water:

• First plateau at 0°C (melting — ice to water)
• Second plateau at 100°C (boiling — water to steam)

Water as an Example

Water is the most important and most studied substance on Earth:

Change	Temperature at standard pressure
Melting point (ice → water)	0°C
Boiling point (water → steam)	100°C

Why does ice float?

Most substances are denser as solids than as liquids — particles pack more closely in the solid state. Water is unusual: when it freezes, hydrogen bonds force water molecules into a hexagonal lattice that is actually less dense than liquid water. Ice floats on water, which is crucial for life — lakes freeze from the top down, allowing aquatic life to survive under the ice.

Diffusion

Diffusion is the net movement of particles from a region of high concentration to a region of low concentration — down the concentration gradient. It occurs due to the random motion of particles.

• Occurs in gases and liquids (not solids — particles cannot move freely)
• Gases diffuse faster than liquids because particles move more rapidly and forces between them are weaker
• Higher temperature increases diffusion rate (particles have more energy)
• Smaller particles diffuse faster than larger ones
• Diffusion continues until the concentration is equal throughout (equilibrium)

Examples:

• Smell of perfume spreading across a room (diffusion through air)
• Food colouring spreading through water without stirring
• Oxygen diffusing from alveoli into blood in the lungs
• Carbon dioxide entering a leaf through stomata for photosynthesis

Worked Example — Heating Curve

A student heats 100 g of ice from -20°C and records the temperature every minute. Describe what happens at each stage:

1. Temperature rises from -20°C to 0°C — ice gaining kinetic energy
2. Temperature stays at 0°C — energy absorbed as latent heat of fusion; ice melting to water
3. Temperature rises from 0°C to 100°C — liquid water gaining kinetic energy
4. Temperature stays at 100°C — energy absorbed as latent heat of vaporisation; water boiling to steam
5. Temperature rises above 100°C — steam gaining kinetic energy

Key Terms

• State of matter — the physical form of a substance: solid, liquid, or gas
• Phase change — a change from one state of matter to another
• Melting point — the temperature at which a solid becomes a liquid
• Boiling point — the temperature at which a liquid becomes a gas throughout
• Evaporation — change from liquid to gas at the surface only; can occur below boiling point
• Latent heat — energy absorbed or released during a phase change with no temperature change
• Diffusion — movement of particles from high to low concentration
• Sublimation — change directly from solid to gas (e.g., dry ice — solid CO2)

Common Mistakes

• Saying temperature rises continuously during melting — it stays constant until the phase change is complete
• Confusing evaporation (surface only, any temperature) with boiling (throughout liquid, at boiling point)
• Thinking diffusion only occurs in gases — it also occurs in liquids, just more slowly
• Saying ice floats because it is lighter — it floats because it is less dense (same mass occupies more volume)

Tips and Tricks

• Phase changes: when energy is added and temperature stays flat on a graph, a phase change is happening
• Solid = low energy, regular arrangement; Gas = high energy, random and spread out
• Diffusion is always down the concentration gradient (high to low) — like spreading from a crowded to an empty space
• Water's unusual density behaviour is why ice floats — important for aquatic ecosystems in winter