Changes of state

• Liquids


  • The particles are closely packed
  • The particles can flow over one another
  • No definite shape – they are able to flow and will take the shape of a container
  • Have a definite volume
• Changes of state between solids, liquids, and gases

  • Melting - When a solid turns into a liquid
  • Boiling - When a liquid turns into a gas (evaporating)
  • Condensing - When a gas turns into a liquid
  • Freezing - When a liquid turns into a solid
  • Subliming - When a solid turns into a gas
• Solid

  • Molecules are very close together and arranged in a regular pattern
  • Molecules vibrate about fixed positions
• Solids


  • The particles are closely packed
  • The particles vibrate about fixed positions
  • Have a definite shape (they are rigid)
  • Have a definite volume
• The temperature of the material is related to the average kinetic energy of its particles
• Gas


  • Molecules are widely separated - about 10 times further apart in each direction
  • Molecules move about randomly at high speeds
• When a substance changes state, the number of molecules in that substance doesn’t change and so neither does its mass. The only thing that changes is its energy. Changes of state are reversible physical changes
• Heating a system

  Changes the energy stored in a system by increasing the kinetic energy of its particles
• Matter can exist in one of three different states: solid, liquid, or gas
• Gases

  • The particles are far apart
  • The particles move randomly
  • No definite shape – they will take the shape of their container
  • No fixed volume – if placed in an evacuated container they will expand to fill the container
  • Highly compressible due to large gaps between particles and easier to push particles closer together than in solids or liquids
• Liquid

  • Molecules are still close together (no gaps) but are no longer arranged in a regular pattern
  • Molecules are able to slide past each other
• Temperature

  Related to the average kinetic energy of the molecules
• Gas molecules move faster with higher kinetic energy as the container is heated up, increasing the internal energy of the system
• Apparatus used to heat ice and measure its temperature as it melts


  1. Place the ice cubes in the beaker (about half full)

  2. Place the thermometer in the beaker

  3. Place the beaker on the tripod and gauze and slowly start to heat it using the bunsen burner

  4. Take regular temperature measurements as the substance changes state (from solid to liquid)
• Heating a system will always increase the energy stored within the system, leading to either an increase in temperature or a change in state
• The higher the temperature, the higher the average kinetic energy of the molecules and vice versa
• Heating a system will change the energy stored in a system by increasing the kinetic energy of its particles
• The total energy within the beaker decreases due to evaporation causing the most energetic particles to leave the beaker
• Increase in kinetic energy

  1. Causes the temperature of the system to increase

  2. Produces a change of state (solid to liquid or liquid to gas)
• When a student measures the mass of a beaker of water twice, leaving 24 hours between the readings, and notices a decrease in mass, the correct conclusion is that the water evaporated
• Equipment List

  • Thermometer = 0.1 °C
  • Stopwatch = 0.1 s
• No mass is lost during evaporation; it is only changed from a liquid to gas state
• The density of water in the air increases when additional water molecules are added to the air without a significant change in the volume of the air
• Core Practical: Investigating Changes of State

  Investigate how the temperature of ice varies when it changes state from a solid to a liquid
• Random Errors

  1. Ensure there are enough ice cubes to surround the thermometer in the beaker and only begin the experiment when the temperature is below 0 °C

  2. This is to ensure readings of temperature are as accurate as possible
• Results table

  • Example results table is shown below
• Systematic Errors:
• Specific Heat Capacity
• Heat can be transferred to a substance without changing its temperature during changes of state
• If a substance has a high specific heat capacity, it heats up and cools down slowly
• Random Errors:
• If a substance has a low specific heat capacity, it heats up and cools down quickly
• Graph will show regions where

  • The temperature of the ice cubes increases
  • There is no temperature change even though the ice cubes continue to be heated
  • A flat region where the ice is melting from solid to liquid
• Experiment steps

  1. Place the thermometer in the beaker

  2. Place the beaker on the tripod and gauze and slowly start to heat it using the bunsen burner

  3. Take regular temperature measurements as the beaker is heated

  4. Continue temperature measurements whilst the substance changes state from solid to liquid
• Once the ice is fully melted, the temperature of the liquid water begins rising again
• Safety Considerations

  1. Wear goggles while heating water

  2. Place the bunsen burner, with the beaker and tripod, on a heatproof mat to avoid surface damage

  3. Stand up during the whole experiment to react quickly to any spills
• Calculating Specific Heat Capacity
• Plot a graph of the temperature against time
• Safety Considerations
• Thermal energy supplied during changes of state weakens the bonds between particles so they become freer to slide around each other