Changes of State (OCR Gateway GCSE Chemistry: Combined Science)

• 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 deposition

Changing 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

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