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 - i.e. when a substance changes from solid to liquid as it absorbs thermal energy
- Freezing - i.e. when a substance changes from liquid to solid as it releases thermal energy
- Vaporisation (or boiling) - i.e. when a substance changes from liquid to gas as it absorbs thermal energy
- Condensation - i.e. 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:
- Solid ice melting into liquid water at 0 °C
- Liquid water boiling and changing 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
