2.1 - States of Matter & Changing State

The particle model of solid, liquid and gas

• Everything is made of tiny particles (atoms, ions or molecules), drawn as small solid spheres.
• The state of a substance - solid, liquid or gas - depends on how strongly its particles are held together.

Solids

• Particles are held close together in fixed positions by strong forces of attraction, in a regular pattern.
• Particles can't move from their positions, so a solid has a definite shape and definite volume.
• Particles vibrate about their fixed positions - the hotter the solid, the more they vibrate, so solids expand slightly on heating.

Liquids

• Forces of attraction are weaker than in a solid.
• Particles are free to move past one another, so a liquid has no fixed shape and flows to fill the bottom of its container.
• Particles stay close together, giving a definite volume.
• Particles move randomly - faster as the liquid heats up, so liquids expand slightly on heating.

Gases

• Forces of attraction are so weak that particles are essentially free of each other.
• Particles move randomly in straight lines at high speed, only interacting when they collide, and spread far apart.
• A gas fills whatever shape and volume its container offers.
• Heating makes the particles move faster, increasing either volume or pressure.

Changes of state

• Melting - Solid to liquid
• Freezing - Liquid to solid
• Boiling or evaporating - Liquid to gas (evaporation happens only at the surface, below the boiling point)
• Condensing - Gas to liquid
• Subliming (sublimation) - Solid straight to gas, skipping the liquid stage (e.g. solid iodine turning directly into a purple gas)

How heating and cooling drive changes of state

• Heat a solid and its particles vibrate more, weakening the forces holding them in place. At the melting point, they have enough energy to break free and form a liquid.
• Heat the liquid further and the particles move faster. At the boiling point, they overcome the forces of attraction completely and form a gas.
• Cooling runs this in reverse: particles lose energy, forces between them strengthen, a gas condenses at its boiling point, and a liquid freezes at its melting point.

Why substances melt and boil at different temperatures

• The energy needed for a change of state depends on the strength of the forces between particles.
• Weak forces need little energy to overcome, giving low melting and boiling points.
• Strong forces need much more energy to overcome, giving high melting and boiling points.

Heating and cooling curves

• Plotting temperature against time while heating or cooling a substance steadily produces a heating or cooling curve.
• Sloped sections - The substance is warming up or cooling down; temperature changes as particles gain or lose kinetic energy.
• Flat sections - A change of state is taking place; temperature stays constant even though energy is being transferred.

Why temperature stays constant during a change of state

• During melting or boiling, energy transferred to the substance is used to break or weaken the forces of attraction between particles, rather than raising the temperature.
• During freezing or condensing, energy is released as bonds/forces form between particles, keeping the temperature steady until the change of state is complete.

Predicting state from melting and boiling data

• Melting point and boiling point are bulk properties that depend on how large numbers of particles interact. A single particle does not have a melting point.
• Given a substance's melting and boiling points, its state at a given temperature is:
  • Below the melting point - Solid
  • Between the melting and boiling points - Liquid
  • Above the boiling point - Gas

---

Worked example - Which substance is a liquid at 150 °C?

Limitations of the simple particle model

• Particles are drawn as solid spheres, but real particles are atoms, ions or molecules with complex shapes and are mostly empty space.
• It assumes spheres are inelastic, ignoring how particles interact during collisions.
• It does not show the forces between particles, so it cannot show how strong those forces are or why different substances have different melting and boiling points.