Changes of state Knowledge-Based Study

Study notes

1. What is a Change of State?

   Matter commonly exists in three primary physical states: solid, liquid, and gas. A change of state, or phase transition, is a physical process in which a substance changes from one physical state to another without altering its chemical composition.

   During any phase change:

   • The chemical identity of the molecules remains identical. For example, ice, liquid water, and steam are all composed of $\text{H}_2\text{O}$ molecules.
   • The physical arrangement, spacing, and movement of the particles change significantly.
   • Mass is strictly conserved throughout the entire transition.

   Solid to Liquid and Liquid to Solid

   The transition between the solid and liquid states is governed by the addition or removal of thermal energy:

   • Melting: When thermal energy is supplied to a solid, the kinetic energy of its particles increases, causing them to vibrate more rapidly. At a characteristic temperature known as the melting point, the particles acquire sufficient energy to break free from their rigid, fixed lattice positions, transitioning into a flowing liquid state.
   • Freezing (Solidification): When heat is removed from a liquid, the particles lose kinetic energy and slow down. Attractive intermolecular forces pull them into regular, fixed positions at the freezing point, converting the liquid into a solid.

   For a pure substance at a given pressure, the melting point and freezing point occur at exactly the same temperature.

   Liquid to Gas and Gas to Liquid

   Transitions between liquids and gases involve large changes in volume and particle spacing:

   • Vaporisation: The conversion of a liquid into a gas. This occurs through two distinct processes:
     • Evaporation: Occurs only at the surface of a liquid at any temperature below the boiling point. Fast-moving particles near the surface overcome attractive forces and escape.
     • Boiling: Occurs throughout the entire volume of the liquid when its vapor pressure equals the external atmospheric pressure, occurring at a specific boiling point.
   • Condensation: When gas particles cool down, their kinetic energy decreases. As they draw closer together, intermolecular forces bind them into a liquid.

   Direct Transitions Between Solid and Gas

   Under certain pressure and temperature conditions, substances can transition directly between solid and gaseous phases without ever passing through an intermediate liquid state:

   • Sublimation: The direct change from a solid to a gas upon absorbing thermal energy. Common examples include dry ice (solid carbon dioxide) at standard atmospheric pressure and iodine crystals when gently warmed.
   • Deposition (or Desublimation): The reverse process, where a gas releases thermal energy and turns directly into a solid. A natural everyday example is the formation of frost on cold surfaces during freezing winter nights.

   Latent Heat and Temperature Plateaus

   A crucial principle in thermodynamics is that the temperature of a pure substance remains constant while it undergoes a change of state, provided the external pressure remains constant.

   When a heating or cooling curve is plotted:

   • Temperature Plateaus: During melting or boiling, thermal energy added does not raise the temperature (kinetic energy) of the particles. Instead, this energy is absorbed as latent heat to overcome intermolecular forces and increase potential energy.
   • Latent Heat of Fusion: The thermal energy required per unit mass to change a substance from solid to liquid at its melting point.
   • Latent Heat of Vaporisation: The thermal energy required per unit mass to change a substance from liquid to gas at its boiling point.

   Once the transition is complete, any continued supply of heat resumes raising the substance's temperature.