Cooling Curve


The graph above shows the cooling curve of a substance.

P
  • The substance exists in gaseous state.
  • The particles have very high energy and are moving randomly.
  • The intermolecular forces between the particles are very weak and can be ignored.
P-Q
  • The substance is in gaseous state.
  • The particles lose kinetic energy during cooling, the particles getting closer to each other and the temperature drops.
Q
  • The substance still exists as a gas.
  • As the molecules are close enough, stronger forces of attraction result in forming of intermolecular bonds.
  • The gas begins to condense and become liquid.
Q-R
  • The process of condensation going on.
  • Stronger bonds form as gas changes into liquid.
  • The substance exists in both gaseous and liquid states.
  • The temperature remains unchanged.
  • This is because the energy produced during the formation of bonds is equal to the heat energy released to the surroundings during cooling.
  • This constant temperature is the boiling point.
  • The heat energy that releases during this condensation process is called the latent heat of vaporization.
R
  • The substance exists only in liquid state as all the gas particles have condensed into liquid.
R-S
  • The substance exists as a liquid.
  • As the temperature falls, the naphthalene molecules lose heat energy. Their movement shows down and they move closer to each other.
S
  • The substance still in liquid state.
  • The particles have very little energy and begin to move closer towards one another as it starts to freeze into solid.
S-T
  • The liquid is changing into solid form.
  • Molecules rearrange to form the molecular arrangement of a solid.
  • The substance exists as both liquid and solid.
  • The temperature remains constant until all the liquid changes to solid.
  • This is because the energy released is the same as the energy lost to the surroundings during cooling.
  • This constant temperature is the freezing point.
  • The heat energy that releases during this freezing process is called the latent heat of fusion.
T
  • All the liquid freezes into solid. The particles are now closely packed in an orderly manner.
T-U
  • Once all the liquid has become solid, the temperature falls once again until it reaches room temperature. The substance is in the solid state here.
U
  • The substance reaches room temperature and remain at this temperature as long as the room temperature remain the same.

Cooling Process


Cooling Curve


Melting Point, Boiling Point and States of Matter


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