Understanding the States of Matter

Welcome to the foundational chapter of chemistry: the States of Matter!

Why does water exist as solid ice, liquid water, and invisible steam? How does a drop of dye spread through water, or a smell travel across a room? This chapter explains how the tiny particles that make up everything behave based on their arrangement, movement, and energy.

I. The Kinetic Particle Theory

The Kinetic Particle Theory states that all matter is made up of tiny particles (atoms, molecules, or ions) that are in constant motion.

Key Principles:
  • All matter is made of particles.
  • Particles are constantly in motion. Even in a solid, particles vibrate about fixed positions.
  • Forces of attraction exist between particles, pulling them together.
  • Temperature affects kinetic energy: As temperature increases, particles gain kinetic energy and move faster.

II. The Three States of Matter

The physical state of a substance depends on the balance between the forces of attraction between particles and the kinetic energy of the particles.

1. Solids
  • Arrangement: Particles are closely packed in a regular, fixed arrangement (lattice).
  • Movement: Particles vibrate about fixed positions; they cannot move past each other.
  • Energy and Forces: Particles possess the lowest kinetic energy; forces of attraction between particles are very strong.
  • Properties: Fixed shape, fixed volume, high density, and cannot be compressed easily.
2. Liquids
  • Arrangement: Particles are closely packed but arranged irregularly and randomly.
  • Movement: Particles can move and slide past one another.
  • Energy and Forces: Particles have more kinetic energy than in solids; forces of attraction are strong, but weaker than in solids.
  • Properties: No fixed shape (they take the shape of the container), fixed volume, and cannot be compressed easily.
3. Gases
  • Arrangement: Particles are far apart with large spaces between them in a completely random arrangement.
  • Movement: Particles move rapidly and randomly in all directions.
  • Energy and Forces: Particles possess the highest kinetic energy; forces of attraction are negligible (almost zero).
  • Properties: No fixed shape and no fixed volume (expand to fill any container), low density, and easily compressed.

III. Interconversions Between States of Matter

Substances change state when heated or cooled. Changing state changes particle arrangement, movement, and energy without changing the chemical identity of the particles.

State Changes:
  • Melting (Solid to Liquid): Heat energy increases particle vibrations until particles gain enough energy to overcome fixed positions and slide past each other.
  • Boiling / Evaporating (Liquid to Gas): Particles gain sufficient kinetic energy to overcome all intermolecular forces of attraction and move far apart.
  • Freezing (Liquid to Solid): Cooling removes energy, particles slow down, and attractive forces pull them into a fixed, regular lattice.
  • Condensation (Gas to Liquid): Cooling slows gas particles down, allowing forces of attraction to bring them close together into a liquid.
  • Sublimation (Solid to Gas): A solid changes directly into a gas upon heating without entering the liquid state (e.g., solid carbon dioxide or iodine).
  • Deposition (Gas to Solid): A gas changes directly into a solid upon cooling.
Boiling vs Evaporation:
Boiling occurs at a specific fixed temperature (the boiling point) throughout the entire liquid.
Evaporation occurs at any temperature below the boiling point and happens only at the liquid surface.

IV. Evidence for Particles: Diffusion and Dilution

1. Dilution of Coloured Solutions

When a small crystal of purple potassium manganate(VII) is placed in water, the purple colour slowly spreads out. If more water is added (dilution), the colour spreads further and becomes paler, but remains evenly distributed throughout the entire liquid.

Explanation: The crystal dissolves into millions of tiny individual particles that spread out randomly among the water particles. This proves that matter is made of tiny particles and that particles in a liquid are constantly moving.

2. Diffusion of Gases

Diffusion is the spreading out of particles from an area of higher concentration to an area of lower concentration down a concentration gradient, due to their random motion.

  • Diffusion in Gases: Very rapid because gas particles move fast and have large spaces between them.
  • Diffusion in Liquids: Slower than in gases because liquid particles are closely packed and constantly collide with solvent particles.
  • Effect of Molecular Mass: Lighter particles (lower relative molecular mass, \(M_r\)) travel and diffuse faster than heavier particles at the same temperature. For example, ammonia gas (\(M_r = 17\)) diffuses faster than hydrogen chloride gas (\(M_r = 36.5\)).

V. Solutions and Solubility

Key Solution Definitions:
  • Solute: The substance that dissolves in a liquid (e.g., salt or sugar).
  • Solvent: The liquid in which the solute dissolves (e.g., water).
  • Solution: The homogeneous mixture formed when a solute dissolves in a solvent.
  • Saturated Solution: A solution that contains the maximum amount of dissolved solute at a specific temperature, such that no more solute can dissolve.
Solubility and Solubility Curves (Paper 2 Only)

Solubility is defined as the mass of solute (in grams) that will dissolve in \(100\text{ g}\) of solvent to form a saturated solution at a specified temperature.

\[\text{Solubility} = \frac{\text{mass of solute (g)}}{\text{mass of solvent (g)}} \times 100\]

Solubility Curves: A graph showing how solubility changes with temperature.

  • For most solid solutes, solubility increases as temperature increases.
  • If a saturated solution at a high temperature is cooled, the solubility decreases and excess solute crystallises out of the solution.