Melting

• Melting is when a solid changes into a liquid

• Requires heat energy which transforms into kinetic energy, allowing the particles to move

• Occurs at a specific temperature known as the melting point (m.p.) 

Boiling

• Boiling is when a liquid changes into a gas

• 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 within the liquid

• Occurs at a specific temperature known as the boiling point (b.p.)

Freezing

• Freezing is when a liquid changes into a solid

• 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

• Requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature 

Evaporation

• Evaporation occurs when a liquid changes into a gas and occurs over a range of temperatures

• Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquid's surface at low temperatures, below the b.p. of the liquid

• The larger the surface area and the warmer the liquid surface, the more quickly a liquid can evaporate

Condensation

• Condensation occurs when a gas changes into a liquid on cooling and it takes place over a range of temperatures

• When a gas is cooled its particles lose energy and when they bump into each other they lack the energy to bounce away again, instead they group together to form a liquid

Sublimation

• Sublimation occurs when a solid changes directly into a gas

• This only happens to a few solids, such as iodine or solid carbon dioxide

• The reverse reaction also happens and is called desublimation or deposition

  
  

Exam Tip

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

Sublimation is no longer part of the syllabus, but you can see how it fits in on the interconversion diagram with the other state changes.