Melting & Boiling (Cambridge International IGCSE Physics)

Fixed points of water

Spec point — 2.2.4 · Know the melting and boiling temperatures for water at standard atmospheric pressure

• 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 standard atmospheric pressure

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

Melting & boiling

Spec point — 2.2.4 · Describe melting and boiling in terms of energy input without a change in temperature

• While a substance is changing state(e.g. melting or boiling):

  • The temperature of the substance does not change
  • Energy is still being transferred to or away from the thermal energy store of the substance

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

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 energy of the molecules, not the kinetic energy

  • 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

• In ice, the molecules are held in a regular lattice and can only vibrate

• After melting, the arrangement becomes irregular and random, and the molecules can move around and slide over one another

Exam Hint When a question asks about arrangement, describing the spacing of molecules will not gain the mark. You must explicitly contrast the regular solid arrangement with the irregular liquid arrangement.

Boiling

• One major difference between melting and boiling (sometimes referred to as vaporisation) is that boiling occurs at a higher temperature

  • 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 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 energy of the molecules, not the kinetic energy

  • 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

Condensation & solidification

Spec point — 2.2.4 · Describe condensation and solidification in terms of particles

• 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, increasing the internal energy of the molecules
  • Cooling is when energy is transferred away from the system (or dissipated to the surroundings), decreasing the internal energy of the molecules

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 energy of the particles, the kinetic energy 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 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 energy of the particles, the kinetic energy is unchanged, so temperature remains constant through this process