• The three states of matter are solids, liquids, and gases

• A substance can usually exist in all three states, dependent on temperature (and pressure)

• Different state changes occur at the melting point and at the boiling point depending on whether the substance is heating up or cooling down

  • At the melting point

    • Melting (solid → liquid) when heating up

    • Freezing (liquid → solid) when cooling down

  • At the boiling point

    • Boiling (liquid → gas) when heating up

    • Condensing (gas → liquid) when cooling down

• Individual atoms themselves do not share the same properties as bulk matter

• The three states of matter can be represented by a simple model

  • In this model, the particles are represented by small solid spheres

• The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles

  • The stronger the forces of attraction, the more energy is needed to overcome them for a state change to occur

  • Therefore, the stronger the forces between the particles the higher the melting point and boiling point of the substance

  
  

• When matter changes from one state to another due to changes in temperature or pressure, the change is called an interconversion of state

• It is a physical change involving changes in the forces between the particles of the substances, the particles themselves remain the same, as do the chemical properties of the substance

• Physical changes are relatively easy to reverse as no new substance is formed during interconversions of state

• The interconversions have specific terms to describe them:

Melting

• Melting is when a solid changes into a liquid

• The process requires heat energy which transforms into kinetic energy, allowing the particles to move

• It occurs at a specific temperature known as the melting point which is unique to each pure solid 

Boiling

• Boiling is when a liquid changes into a gas

• This 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 from within the liquid

• It occurs at a specific temperature known as the boiling point 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 0 ºC

  
  

• It requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature that 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 surface at low temperatures, below the boiling point of the liquid

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

• Evaporation occurs over a range of temperatures, but heating will speed up the process as particles need energy to escape from the surface

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 grouping together to form a liquid

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 called desublimation or deposition