State Changes
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.) which is unique to each pure solid
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.) which is unique to each pure liquid
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 0ºC
- 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 changes into a gas. 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
- No heat is required and evaporation occurs over a range of temperatures
Condensation
- When a gas changes into a liquid, usually on cooling. 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
- No energy is required for condensation to occur and it takes place over a range of temperatures
Sublimation
- When a solid changes directly into a gas
- This happens to only a few solids such as iodine or solid carbon dioxide
- The reverse reaction also happens and is also called sublimation (sometimes called deposition or desublimation)
- Sublimation occurs at a specific temperature which is unique for a pure substance
Interconversion of solids, liquids and gases
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.
Gaseous Particles
- Gaseous particles are in constant and random motion
- An increase in temperature increases the kinetic energy of each particle, as the thermal energy is transformed to kinetic energy, so they move faster
- Decreasing the temperature has the opposite effect
- The pressure that a gas creates inside a closed container is produced by the gaseous particles hitting the inside walls of the container. As the temperature increases, the particles in the gas move faster, impacting the container’s walls more frequently
- Therefore an increase in temperature causes an increase in pressure
Moving particles of gas colliding with each other and the container walls
