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2.3.2 Changes of State

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Changes of State

  • The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles
  • The stronger the forces of attraction, the more energy that is needed to overcome them for a state change to occur
  • Therefore, the stronger the forces between the particles the higher the melting point and boiling point of the substance
  • For example, simple molecules have weak intermolecular forces that need little energy to break resulting in low melting and boiling points
  • Other structures, such as giant ionic lattices and macromolecular structures have very strong bonds which need lots of energy to break, resulting in high melting and boiling points

Melting

  • Melting is when a solid (s) changes into a liquid (l)
  • The process requires heat energy which transforms into kinetic energy, allowing the particles to move
  • It occurs at a specific temperature known as the melting point which is unique to each pure solid 

Boiling

  • Boiling is when a liquid (l) changes into a gas (g)
  • This requires heat which causes bubbles of gas to form below the surface of a liquid, allowing for liquid particles to escape from the surface and from within the liquid
  • It occurs at a specific temperature known as the boiling point which is unique to each pure liquid 

Freezing

  • Freezing is when a liquid (l) changes into a solid (s)
  • This is the reverse of melting and occurs at exactly the same temperature as melting, hence the melting point and freezing point of a pure substance are the same
    • Water for example freezes and melts at 0 ºC
  • It requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature which is unique for each pure substance 

Evaporation

  • When a liquid (l) changes into a gas (g)
  • Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquids surface at low temperatures, below the boiling point of the liquid
  • The larger the surface area and the warmer the liquid/surface, the more quickly a liquid can evaporate
  • Evaporation occurs over a range of temperatures, but heating will speed up the process as particles need energy to escape from the surface

Condensation

  • When a gas (g) changes into a liquid (l), usually on cooling
  • When a gas is cooled its particles lose energy and when they bump into each other, they lack energy to bounce away again, instead grouping together to form a liquid

Sublimation

  • When a solid (s) changes directly into a gas (g)
  • This happens to only a few solids, such as iodine or solid carbon dioxide
  • The reverse reaction also happens and is called desublimation or depositionChanging state, IGCSE & GCSE Chemistry revision notes

Changing states

Predicting States

  • The physical state of a substance under certain conditions can be predicted from a given set of data
  • Normally you are given melting and boiling point data for a substance and asked to predict its physical state in specified conditions.
  • At temperatures below the melting point:
    • The substance will be in the solid state
  • At temperatures between the melting point and the boiling point:
    • The substance will be in the liquid state
  • At temperatures above the boiling point:
    • The substance will be in the gas state

Worked example

The table below indicates melting and boiling point data for four different substances named A, B, C and D.Melting Point Table, downloadable IGCSE & GCSE Chemistry revision notes

Predict the states of the following substances:

  • Substance A at -150 ºC
  • Substance B at 50 ºC
  • Substance C at 1400 ºC
  • Substance D at 400 ºC

Answer:

    • A boils at temperatures above -173 ºC so at -150 ºC A is a gas
    • B melts at 1736 ºC so at 50 ºC it is a solid
    • C melts at 1105ºC and boils at 1450 ºC so at 1400 ºC it is a liquid
    • D melts at 650 ºC so at 400 ºC it is a solid

Exam Tip

Questions on the particle theory of matter show interconversion of states with a reversible arrow: ⇌

This means that the process can go forwards and backwards. Read the question carefully and pick the direction of the change in state that the question refers to.

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