Changes of State (DP IB Physics: HL): Revision Note

Changes of State

• A change of state, or phase change, happens whenever matter changes from one phase (solid, liquid or gas) into another

  • During a phase change, thermal energy is transferred to or from a substance

• This energy transfer does not change the temperature of the substance undergoing the phase change, meaning

  • the thermal energy provided (or removed) does not affect the kinetic energy of the molecules within the substance

  • only the potential energy (i.e. the spacing between the atoms or molecules) is affected

• The four main phase changes are:

  • Melting - when a substance changes from solid to liquid as it absorbs thermal energy

  • Freezing - when a substance changes from liquid to solid as it releases thermal energy

  • Vaporisation (or boiling) - when a substance changes from liquid to gas as it absorbs thermal energy

  • Condensation - when a substance changes from gas to liquid as it releases thermal energy

Phase changes of water

• Each substance has its own melting (or freezing) and boiling points

  • For example, the freezing point of water is 0 °C, and its boiling point is 100 °C

• Possible phase changes of water include when:

  • solid ice melts into liquid water at 0 °C

  • liquid water boils and changes into gaseous water vapour at 100 °C

• Both these changes happen when thermal energy is absorbed

  • If thermal energy is released from water vapour at 100 °C, it condenses back into water

  • If water continues to release thermal energy, it cools down until it reaches 0 °C and freezes into ice

• Melting and freezing happen at the melting / freezing point of a substance

• Vaporisation and condensation happen at the boiling point of a substance