P1.2 CHANGES OF STATE

• • When a substance changes state, the number of molecules in that substance doesn’t change and so neither does its mass
  • This is known as conservation of mass
• • During a change of state the internal energy does change
  • Internal energy increases with increase in temperature (from solid to gas)
• Melting - When a solid turns into a liquid
• • Evaporating When a liquid turns into a gas
• Freezing - When a liquid turns into a solid
• • Subliming When a solid turns straight into a gas
• • Changes of state are physical changes
  • Physical changes are reversible
  • This means that the material recovers its original properties if the change is reversed
• • In a solid:
  • The molecules are very close together and arranged in a regular pattern
  • The molecules vibrate about fixed positions
• • In a liquid:
  • The molecules are still close together (no gaps) but are no longer arranged in a regular pattern
  • The molecules are able to slide past each other
• • In a gas:
  • The molecules are widely separated.
  • The molecules move about randomly at high speeds
• • The molecules within a substance possess two forms of energy:
  • Kinetic energy (due to their random motion / vibration)
  • Potential energy (due to their position relative to each other)
  • Together, these two form the total energy that makes up the internal energy of the system
• • Internal energy is defined as:

  The total energy stored inside a system by the particles that make up the system due to their motion and positions
• Heating a system changes a substance's internal energy by increasing the kinetic energy of its particles
• • The higher the temperature, the higher the kinetic energy of the molecules and vice versa
  • This means they move around faster
• • This increase in kinetic energy (and therefore internal energy) can:
  • Cause the temperature of the system to increase
  • Or, produce a change of state (solid to liquid or liquid to gas)
• • When a substance reaches a certain temperature, the kinetic energy of the molecules will stop increasing and the energy will go into increasing its potential energy instead
  • This breaks the bonds between the molecules, causing them to move further apart and leads to a change of state
  • For example, liquid to gas
• • When a substance changes its state:
  • The potential energy of the molecules increases, breaking the bonds between them and becoming further apart
  • The kinetic energy remains the same, meaning that the temperature will remain the same, even though the substance is still being heated
• • If the temperature of the system increases, the increase in temperature of this system depends on:
  • The mass of the substance heated
  • The type of material
  • The energy input to the system
• • The specific heat capacity of a substance is defined as:

  The amount of energy required to raise the temperature of 1 kg of the substance by 1 °C
• • If a substance has a low specific heat capacity, it heats up and cools down quickly (ie. it takes less energy to change its temperature)
  • If a substance has a high specific heat capacity, it heats up and cools down slowly (ie. it takes more energy to change its temperature)
• Specific Latent Heat

  • Energy is required to change the state of a substance
  • This energy is known as latent heat
• • The specific latent heat is defined as:

  The amount of thermal energy required to change the state of 1 kg of a substance with no change in temperature
• • There are two types of specific latent heat:
  • Specific latent heat of fusion (solid to liquid and vice versa)
  • Specific latent heat of vaporisation (liquid to gas and vice versa)
• • The specific latent heat of fusion is defined as:

  The thermal energy required to convert 1 kg of solid to liquid with no change in temperature

  • The latent heat of fusion is the energy needed to break the bonds between the molecules
• • The specific latent heat of vaporisation is defined as:

  The thermal energy required to convert 1 kg of liquid to gas with no change in temperature

  • The latent heat of vaporisation is the energy needed by the particles to break away from their neighbouring particles in the liquid
• • Specific heat capacity and specific latent heat are slightly different
  • Specific heat capacity is used for a change in temperature in the same state
  • Specific latent heat is used for a change in state but no change in temperature