Melting & Boiling (CIE IGCSE Physics)
Revision Note
Author
Lindsay GilmourExpertise
Physics
Fixed Points of Water
- The melting and boiling points of pure water are known as fixed points
- Ice melts at 0 °C
- Pure water boils at 100 °C
- These are the accepted values for pure water at atmospheric pressure
Ice melts at 0 °C and water boils at 100 °C
Melting & Boiling
- While a substance is changing state, either
- Melting or freezing
- Boiling or condensing
- The substance does not change temperature, even though energy is being transferred to or away from the thermal energy store of the substance
Boiling
- When liquid water is heated by adding thermal energy (say from the gas flame or kettle element), the temperature of the water rises until the water boils
- At the boiling point, even if more thermal energy is added, the liquid water does not get any hotter
- This means that the internal energy is not rising
- The additional thermal energy goes into overcoming the intermolecular forces between the molecules of water
- As the forces are overcome, the liquid water becomes water vapour (steam)
- This is evaporation or vaporisation; the water is now a gas
- The process is repeated backwards for cooling as energy is transferred away
- A gas turns back into liquid through condensation
Melting
- When solid water (ice) is heated by adding thermal energy (from the surroundings, or a flame), the ice melts
- At the melting point, even if more thermal energy is added, the solid water does not get warmer
- This means that the internal energy is not rising
- The additional thermal energy goes into overcoming the intermolecular forces between the molecules of the solid ice
- As the forces are overcome, the solid water becomes liquid
- This is melting; the ice is now a liquid
- The process is repeated backwards for cooling as heat is transferred away
- A liquid turns back into a solid through freezing
Condensation & Solidification
- 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 system and the kinetic energy of the molecules increases (red arrows to the right)
- Cooling is when energy is transferred away from the system (or dissipated to the surroundings) and the kinetic energy of the molecules decreases (blue arrows to the left)
Condensation
- The particle diagrams next to the graph show that as a gas condenses into a liquid
- The gas has already lost heat energy (cooled down)
- The particles lose kinetic energy and move more slowly
- They no longer have enough energy to overcome the intermolecular forces of attraction between molecules
- The particles get closer together
- They only have enough energy to flow over one another
- The gas has condensed into a liquid with no change of temperature
Solidification
- The particle diagrams next to the graph show that as a liquid solidifies into a solid
- The liquid has already lost heat energy (cooled down)
- The particles lose kinetic energy and move more slowly
- They no longer have enough energy to overcome the intermolecular forces of attraction between molecules
- The particles get closer together
- They only have enough energy to vibrate about their fixed position
- The liquid has solidified into a solid with no change of temperature
Heating/cooling curve of a substance showing the energy changes as temperature is increased/decreased
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