Melting & Boiling (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

• The melting and boiling points of pure water are known as fixed points
  • Ice melts at 0 °C
  • Pure water boils at 100 °C

• These are the accepted values for pure water at atmospheric pressure

Ice melts at 0 °C and water boils at 100 °C 

• While a substance is changing state (e.g. melting or boiling) the substance does not change temperature, even though energy is being transferred to or away from the thermal energy store of the substance
• One major difference between melting and boiling is that boiling occurs at higher energy
  • While melting results in particles being able to flow and move more freely, during boiling enough energy is transferred such that the intermolecular forces can be completely overcome

When the substance is changing state, the temperature remains constant as shown by the horizontal regions of this graph

Boiling

• When liquid water is heated by adding thermal energy (say from a gas flame or kettle element), the temperature of the water rises until the water boils
  • At the boiling point, even if more thermal energy is added, the temperature of the liquid water does not increase
  • This means that the internal energy is not rising
• The additional thermal energy goes into overcoming the intermolecular forces between the molecules of water
  • As the forces are overcome, the liquid water becomes water vapour (steam)
  • This is evaporation or vaporisation; the water is now a gas

Melting

• When solid water (ice) is heated by adding thermal energy, the temperature of the ice increases up until the melting point
  • At the melting point, even if more thermal energy is added, the ice does not get warmer
  • This means that the internal energy is not rising

• The additional thermal energy goes into overcoming the intermolecular forces between the molecules of the ice
  • As the forces are overcome, the solid water becomes liquid
  • This is melting; the ice is now a liquid

• Changes of state depend on whether energy is being transferred to or away from the system
  • Heating is when energy is transferred to the system and the kinetic energy of the molecules increases (red arrows to the right)
  • Cooling is when energy is transferred away from the system (or dissipated to the surroundings) and the kinetic energy of the molecules decreases (blue arrows to the left)

Condensation

• When a gas cools, energy is transferred away from the system and kinetic energy decreases until the temperature reaches boiling point
  • At boiling point, energy transferred away from the system reduces its potential energy
  • The particles no longer have enough energy to overcome the intermolecular forces of attraction
  • They only have enough energy to flow over one another
• The gas has condensed; it is now a liquid
  • As the energy has been transferred away from the potential store of the particles, the energy in the kinetic store is unchanged, so temperature remains constant through this process

Solidification (freezing)

• When a liquid cools, energy is transferred away from the system and kinetic energy decreases until the temperature reaches melting point
  • At melting point, energy transferred away from the system reduces its potential energy
  • The particles no longer have enough energy to overcome the intermolecular forces of attraction
  • They are now low enough in energy to be bound to each other and can only vibrate around a fixed point
• The liquid has solidified or frozen; it is now a solid
  • As the energy has been transferred away from the potential store of the particles, the energy in the kinetic store is unchanged, so temperature remains constant through this process