State changes occur when:

• Solids become liquids

• Liquids become gases

• Gases become liquids

• Liquids become solids

• Each state change requires a change in the energy, arrangement and movement of the particles

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.) 

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 

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.)

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