States of Matter

Melting

• Solid changes to liquid
• Particles gain kinetic energy and escape their fixed lattice positions, starting to slide
• Happens at one specific temperature, the melting point (m.p.)
• A pure substance has a sharp m.p. (a standard purity test)

Freezing

• Liquid changes to solid (the reverse of melting)
• Particles lose kinetic energy and lock back into a fixed arrangement
• For a pure substance, freezing point = melting point
  • Water freezes / ice melts at 0 °C

Boiling

• Liquid changes to gas at one specific temperature, the boiling point (b.p.)
• Vapour bubbles form throughout the bulk of the liquid
• The temperature of the liquid stays at the b.p. while boiling continues
• Continuous heating is needed to maintain the change

Evaporation

• Liquid changes to gas at any temperature below the b.p.
• A surface-only process: only particles at the liquid surface escape
• The fastest-moving surface particles have enough kinetic energy to break free
• Speeds up when:
  • surface area is larger
  • temperature is higher
  • a draught removes the escaping vapour

Condensation

• Gas changes to liquid on cooling
• Particles lose kinetic energy, and attractive forces hold colliding particles together
• Occurs over a range of temperatures, not at one fixed value

Sublimation and deposition

• Sublimation = solid → gas, skipping the liquid state
  • Solid iodine (purple vapour on warming)
  • Solid carbon dioxide ("dry ice")
• Deposition (also "desublimation") = gas → solid, skipping the liquid state
  • Frost forming on cold glass from water vapour