States of Matter (AQA GCSE Chemistry: Combined Science)

• The three states of matter are solids, liquids and gases
• A substance can usually exist in all three states, dependent on temperature (and pressure)
• State changes occur at the melting point (solid to liquid, liquid to solid) and at the boiling point (liquid to gas and gas to liquid)
  • Melting and freezing occur at the melting point
  • Boiling and condensing take place at the boiling point

• 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 the three states of matter

	Solid	Liquid	Gas
Diagram
Arrangement of particles	Regular arrangement	Randomly arranged	Randomly arranged
Movement of particles	Vibrate about a fixed position	Move around each other	Move quickly in all directions
Closeness of particles	Very close	Close	Far apart

• 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 

Melting

• Melting is when a solid changes into a liquid
• Heat / thermal energy absorbed by the particles is transformed into kinetic energy
• This causes the particles to vibrate more and start to move / flow
• Melting happens at a specific temperature, known as the melting point (m.p.) 

Boiling

• Boiling and evaporation are both when a liquid changes into a gas
  • However, there is a key difference between boiling and evaporation
• In boiling, heat / thermal energy causes bubbles of gas to form inside the liquid, allowing for liquid particles to escape from the surface and within the liquid
• Boiling happens 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 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
• Freezing needs a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature 

Evaporation

• 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

• Condensation occurs when a gas changes into a liquid on cooling and 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

• 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

Changing states of matter

State changes require a change in the energy of the particles