Melting & Boiling (CIE IGCSE Physics)

• 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, either
  • Melting or freezing
  • Boiling or condensing

• The substance does not change temperature, even though energy is being transferred to or away from the thermal energy store of the substance

Boiling

• When liquid water is heated by adding thermal energy (say from the 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 liquid water does not get any hotter
  • 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

• The process is repeated backwards for cooling as energy is transferred away
  • A gas turns back into liquid through condensation

Melting

• When solid water (ice) is heated by adding thermal energy (from the surroundings, or a flame), the ice melts
  • At the melting point, even if more thermal energy is added, the solid water 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 solid ice
  • As the forces are overcome, the solid water becomes liquid
  • This is melting; the ice is now a liquid

• The process is repeated backwards for cooling as heat is transferred away
  • A liquid turns back into a solid through freezing

• Heating and cooling graphs are used to summarise:
  • How the temperature of a substance changes when energy is transferred to or away from it
  • Where changes of state occur

• Heating and cooling graphs tend to be the same
  • 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

• The particle diagrams next to the graph show that as a gas condenses into a liquid
  • The gas has already lost heat energy (cooled down)

• The particles lose kinetic energy and move more slowly
  • They no longer have enough energy to overcome the intermolecular forces of attraction between molecules 
  • The particles get closer together
  • They only have enough energy to flow over one another

• The gas has condensed into a liquid with no change of temperature

Solidification

• The particle diagrams next to the graph show that as a liquid solidifies into a solid
  • The liquid has already lost heat energy (cooled down)

• The particles lose kinetic energy and move more slowly
  • They no longer have enough energy to overcome the intermolecular forces of attraction between molecules 
  • The particles get closer together
  • They only have enough energy to vibrate about their fixed position

• The liquid has solidified into a solid with no change of temperature

Heating/cooling curve of a substance showing the energy changes as temperature is increased/decreased