States of Matter (Cambridge (CIE) O Level Chemistry): Revision Note

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.) 

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 °C

• 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 change also happens and is called desublimation or deposition

Interconversion of solids, liquids and gases

• The diagram labels the change from liquid to gas as evaporation, which happens over a range of temperatures at the liquid's surface

  • The same change happening throughout the liquid at its boiling point is called boiling

State Changes & Kinetic Theory

• When substances are heated, the particles absorb thermal energy which is converted into kinetic energy. This is the basis of the kinetic theory of matter

• Heating a solid causes its particles to vibrate more and as the temperature increases, they vibrate so much that the solid expands until the structure breaks and the solid melts

• On further heating, the now liquid substance expands more and some particles at the surface gain sufficient energy to overcome the intermolecular forces and evaporate

• When the b.p. temperature is reached, all the particles gain enough energy to escape and the liquid boils

• These changes in state can be shown on a graph called a heating curve

• Cooling down a gas has the reverse effect and this would be called a cooling curve

• These curves are used to show how changes in temperature affect changes of state

• The horizontal sections occur when there is a change of state but there is no change in temperature

  • The energy being supplied is used to overcome the forces of attraction between the particles, not to increase their kinetic energy

  • The temperature only rises again once the state change is complete

A heating curve showing the states, state changes and temperature changes as time progresses

A cooling curve is like a heating curve, but is the mirror image