Changes of State

• Conservation of Mass:

  • When a substance changes state, the number of molecules in that substance doesn’t change and so neither does its mass
  • This is known as conservation of mass
  • During a change of state, the internal energy does change
  • Internal energy increases with an increase in temperature (from solid to gas)
• Physical vs Chemical Changes:

  • Changes of state are physical changes
  • Physical changes are usually reversible
  • This means that the material recovers its original properties if the change is reversed
  • Physical changes are different to chemical changes because chemical changes are often not reversible
• • In a solid:
  • The molecules are very close together and arranged in a regular pattern
  • The molecules vibrate about fixed positions
• • In a liquid:
  • The molecules are still close together (no gaps) but are no longer arranged in a regular pattern
  • The molecules are able to slide past each other
• • In a gas:
  • The molecules are widely separated - about 10 times further apart in each direction
  • The molecules move about randomly at high speeds
• showing the arrangement and motion of different states of matter. These physical changes are reversible:

  
• There are six changes of state that can occur between solids, liquids and gases: