Lesson 2, Topic 4
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Explaining changes of state (S)

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Freezing and melting / boiling

  • When a solid is heated, its particles gain more energy.

  • This makes the particles vibrate more, which weakens the forces (bonds) that hold the solid together. This makes the solid expand

  • At a certain temperature, the particles have enough energy to break free from their positions. This is called melting and the solid turns into a liquid

  • When a liquid is heated, the particles get even more kinetic energy 

  • This energy makes the particles move faster, which weakens and breaks the bonds holding the liquid together

  • At a certain temperature, the particles have enough kinetic energy to break their bonds. This is called boiling and the liquid turns into a gas.

  • The amount of energy needed to cause a substance to change state depends on the strength of the forces between the particles

Heating and cooling curves

When a change of state happens energy is either released or absorbed

  • When a substance condenses or freezes the kinetic energy in its particles is converted to heat energy and released to the surroundings

  • When a substance melts or boils  heat energy is absorbed from the surroundings and converted to kinetic energy in the particles causing bonds between them to weaken or break

Heating and cooling curves are graphs showing how the temperature changes when a substance is heated or cooled at a steady rate

A-B :

  • An input of energy increases the temperature of the solid

B-C:

  • The energy is being used to reduce the attractive forces between the particles in the solid

  • So is not available to raise the temperature

C-D: 

  • The energy increases the temperature of the liquid

D-E:

  • The energy is being used to overcome the attractive forces between the particles in the liquid

  • So is not available to raise the temperature

E-F:

  • The energy increases the temperature of the gas