States of matter

• • The three states of matter are solids, liquids and gases
  • A substance can usually exist in all three states, dependent on temperature (and pressure)
• • 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 between the particles, the more energy that is needed to overcome them 
  • Therefore, the stronger the forces between the particles the higher the melting point and boiling point of the substance
• • Changing states is a physical change
  • The particles themselves remain the same, it is just the forces between the particles which change 
• • This is the reverse of melting and occurs at the same temperature as melting
  • So, the melting point and freezing point of a pure substance are the same
  • For example, water freezes and melts at 0 ºC
• • This is the reverse of melting and occurs at the same temperature as melting
  • So, the melting point and freezing point of a pure substance are the same
  • For example, water freezes and melts at 0 ºC
• • Evaporation occurs over a range of temperatures
  • It can happen at temperatures below the boiling point of the liquid
• • Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquid's surface at low temperatures
  • The larger the surface area and the warmer the liquid surface, the more quickly a liquid can evaporate
• • Condensation occurs when a gas changes into a liquid on cooling and takes place over a range of temperatures
• State changes require a change in the energy of particles
• • At temperatures below the meltingpoint:
  • The substance will be in the solid state
• • At temperatures between the melting point and the boiling point:
  • The substance will be in the liquid state
• • At temperatures above the boilingpoint:
  • The substance will be in the gas state
• Questions on the particle theory of matter show interconversion of states with a reversible arrow: ⇌