Changes of State (OCR GCSE Combined Science A (Gateway)): Revision Note

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 deposition

  

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