Melting, Boiling & Evaporation (Cambridge O Level 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 Melting and Water Boiling

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

Changing Temperature through State Changes

Energy is transferred to the material over time. This causes an increase in temperature when the material is a solid, liquid or gas, but does not cause an increase in temperature when there is a change of state.

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 Graph

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