Unit 1.2 – Phase changes

In order for a phase change to occur, there must be a change in energy. Heat can be given off by the compound or taken in depending which phase change it will undergo.

Endothermic reaction

When heat or energy from the surrounding enters the system (compound). The result would cause a loss of heat in the surrounding making it cooler than before.

Example of this is when barium hydroxide is reacted with ammonium thiocyanate. The exterior becomes “cold” because heat is taken away from the outside to enter the system for the reaction.

Another example is making ice cream with salt and ice. When salt causes the melting of the ice cubes, the process of melting is an endothermic reaction. The heat from the surrounding area, in this case, the liquid milk is taken away making the liquid milk drop in temperature.  The milk will eventually get below 0C to freeze into ice-cream.

Exothermic reaction

When heat is given off during the reaction to the surrounding area. This results in the warming of the surrounding area.

Example is when you mix sulfuric acid with water. The container / beaker will get warm because heat is given off.


Recall, solids are tightly packed, with high density and the particles vibrate in their own position. If energy is placed into the solid, the particles will gain more energy causing the vibrations to intensify. Once they vibrate faster, it will break apart the intermolecular attraction between the atoms causing the particles to enter the next phase.


  • When energy is added, the particles become more excited and therefore move faster.
  • As they move faster, they collide more, causing the intermolecular forces to break down and a change of state results.
  • The energy being used is called kinetic energy
  • As a solid is heated (energy is absorbed), the forces of attraction between the particles of the crystal are slowly overcome, allowing some particles to move more freely.

Freezing Point

Freezing point is the opposite of melting point where liquid solidifies into a solid. In order for a liquid to change into a solid, the particles must slow down, thus, energy must be “taken out” or leave the system.

  • the temperature at which a liquid solidifies into a solid.
  • the particles begin to slow down and the forces of attraction between the particles, or intermolecular forces begin to increase and take hold. Energy is lost to the surroundings


Vaporization is when liquid is changed into gas. In order for vaporization and evaporation to occur, vapour pressure must be understood and defined.

Vapour pressure varies from liquid to liquid and varies with temperature. Vapour pressure is the pressure the liquid molecules exert to escape to the gas phase. In a closed container, there is not change in pressure inside the system. Thus, there is a dynamic equilibrium of liquid molecules entering the gas phase and gas phase condensing to the liquid phase. This dynamic equilibrium is the equal number of particles changing phases. We don’t normally see water suddenly boil or evaporate because our atmosphere has pressure. Our atmosphere contains air molecules that act as the atmospheric pressure. Therefore, the higher the atmospheric pressure, the more difficult for the liquid to vaporize. However, if the atmospheric were to lower, then there are less air molecules “sitting” on top of the liquid, making it easier for the liquid to vapourize.


Vaporization: is the endothermic (heat is put into the system or object) process by which a liquid changes to a gas or vapour

  • It includes evaporation and boiling.

Evaporation happens below the boiling point while boiling requires temperature (heat)

  • In order to understand evaporation and vapourization, vapour pressure must be understood.
  • In liquids, there is a pressure that is exerted from the molecules.
  • But the molecules (from the liquid) can’t vaporize because there is also air molecules keeping them in the phase.
  • In order for evaporation to occur, the molecules in the liquid must overcome the air molecule or air pressure. Thus, if the air pressure drops, evaporation can occur easier.
  • In boiling, heat is needed. Heat causes the molecules to move faster, thus exceeding the intermolecular forces, boils.


  • Evaporation occurs when some particles on the surface of the liquid have enough energy to overcome the forces of attraction between the liquid particles.
  • Evaporation takes the energy from the surroundings, excites molecules and eventually changes into vapor.

So explain why it feels cold when your body is wet?

Explain the why it feels cold when alcohol is placed on your hand? Also why it disappears faster than water? What can you conclude about the vapour pressure of alcohol compared to water?


  • Condensation: is the exothermic process by which a gas or vapour becomes a liquid
  • Vaporization and Condensation are said to be in a dynamic equilibrium with one another
  • This means that in a closed system,

the rate of vaporization = rate of condensation

  • In other words, the particles escaping as gas equal the particles returning as liquid
  • Therefore, the level of the liquid will not change at constant temperature.

Sublimation: is the endothermic change from a solid to a gas or vapour without passing through the liquid state

ex: dry ice (solid CO2) and solid iodine

Deposition: is the exothermic change directly from gas to solid skipping the liquid state.

ex: hoarfrost