States of Matter (Cambridge O Level Chemistry)

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

Interconversion of solids, liquids and gases

 

• 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 liquids 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

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