Melting, Boiling & Evaporation (Cambridge (CIE) O Level Physics): Revision Note

Fixed Points of Water

• 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 

Melting & Boiling

• 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 (e.g., 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

• The additional thermal energy is transferred to the potential store of the molecules, not the kinetic store

  • This energy goes into overcoming the intermolecular forces between the molecules of water, so the internal energy continues to rise even though the temperature stays constant

  • As the forces are overcome, the liquid water becomes water vapour (steam); the water is now a gas

Melting

• When solid water (ice) is heated by adding thermal energy, the temperature of the ice increases until the melting point is reached

  • At the melting point, even if more thermal energy is added, the temperature of the ice does not increase

• The additional thermal energy is transferred to the potential store of the molecules, not the kinetic store

  • This energy goes into overcoming the intermolecular forces between the molecules of the ice, so the internal energy continues to rise even though the temperature stays constant

  • As the forces are overcome, the solid water becomes liquid; this is melting

Condensation & Solidification

• 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, increasing the internal energy of the molecules (red arrows to the right)

  • Cooling is when energy is transferred away from the system (or dissipated to the surroundings), decreasing the internal energy of the molecules (blue arrows to the left, back along the energy-added axis; a cooling curve plotted against time is read from left to right, with the temperature falling)

Condensation

• When a gas cools, energy is transferred away from the system, and kinetic energy decreases until the temperature reaches the boiling point

  • At the 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

• When a liquid cools, energy is transferred away from the system, and kinetic energy decreases until the temperature reaches the melting point

  • At the 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; 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

Heating / Cooling Graph

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