Heating & Cooling Graphs (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Heating & Cooling Graphs

• Heating and cooling graphs are used to summarise:

  • How the temperature of a substance changes when energy is transferred to or away from it 

  • Where changes of state occur

• Heating and cooling graphs tend to be the same

  • Heating is when energy is transferred to the substance and the kinetic energy of the molecules increases (red arrows to the right)

  • Cooling is when energy is transferred away from the substance (dissipated to the surroundings) and the kinetic energy of the molecules decreases (blue arrows to the left)

Heating

• When energy is transferred to a solid, the temperature starts to increase as the particles gain kinetic energy and vibrate faster 

• When the temperature of the substance reaches its melting point, the energy transferred to the substance is now used to overcome the intermolecular forces of attraction holding the particles in their solid structure

• The temperature of the substance stops increasing 

  • This is melting and will continue until all the particles have reached the liquid state 

• As energy is continually added, the temperature of the liquid starts to increase as the particles gain more kinetic energy and move over one another faster

• When the temperature of the substance reaches its boiling point, the energy transferred to the substance is now used to overcome the intermolecular forces of attraction holding the particles in their liquid structure 

• The temperature of the substance stops increasing 

  • This is boiling or vaporisation and will continue until all the particles have reached the gaseous state

Cooling

• The process is repeated backwards for cooling as energy is transferred away

  • A gas turns back into liquid through condensation

  • A liquid turns back into a solid through freezing

• When energy is transferred away from a gas, the temperature starts to decrease as the particles lose kinetic energy and move slower 

• When the temperature of the substance reaches its boiling point, the energy transferred away from the substance is now taken from the energy used to overcome the intermolecular forces of attraction

• The temperature of the substance stops decreasing

• The particles now succumb to those intermolecular forces of attraction and are held together in their liquid structure 

  • This is condensing and will continue until all the particles have reached the liquid state 

• When energy is transferred away from a liquid, the temperature starts to decrease as the particles lose more kinetic energy and move even slower 

• When the temperature of the substance reaches its melting point, the energy transferred away from the substance is now taken from the energy used to overcome the intermolecular forces of attraction

• The temperature of the substance stops decreasing

• The particles now succumb to those intermolecular forces of attraction and are held together in their solid structure 

  • This is freezing and will continue until all the particles have reached the solid state 

Heating/cooling graph of a substance showing the energy changes as temperature is increased/decreased

The different sections of the graph show:

• ORIGIN to A: Energy transferred to the substance is being used to increase the kinetic energy of the particles while it is a solid

• A to B: Energy transferred to the substance is being used to overcome the intermolecular forces of attraction, increasing the potential energy and melting the substance

• B to C: Energy transferred to the substance is being used to further increase the kinetic energy of the particles while the substance is a liquid

• C to D: Energy transferred to the substance is being used to overcome the intermolecular forces of attraction, further increasing the potential energy and boiling the substance

• D to E: Energy transferred to the substance is being used to further increase the kinetic energy of the particles while the substance is a gas

Pure vs impure substances

• A pure substance changes state at a fixed, constant temperature

• This produces a flat plateau on the heating/cooling graph

• An impure substance has dissolved particles that disrupt the structure

  • So, it melts or boils over a range of temperatures rather than at a single point

• No flat plateau on the graph indicates the substance is impure