Edexcel IGCSE Chemistry: Double Science 复习笔记:1.1.1 The Three States of Matter

Edexcel IGCSE Chemistry: Double Science 复习笔记:1.1.1 The Three States of Matter

The Three States of Matter



  • The three states of matter are solidsliquids 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



Summary of the Properties of Solids, Liquids and Gases





Interconversion Between the States of Matter


  • 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 that 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:


A Summary of State Changes 







  • 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 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 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 which is unique for each pure substance 



  • When a liquid changes into a gas
  • Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquids 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



  • 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 energy to bounce away again, instead grouping together to form a liquid



  • 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






Interconversion between the three states of matter



Exam Tip

Solids, liquids and gases have different physical properties. The difference in these properties comes from differences in how the particles are arranged in each state.