Chemistry and Physics classes often require students to understand the step-by-step processes that occur when a substance is heated or cooled.

 

Use this summary on heating and cooling curves to learn when:

  • melting, boiling/evaporation, sublimation, condensation, freezing/solidification, and deposition take place
  • heat (Joules/Celsius) is absorbed (endothermic) or released (exothermic)
  • temperature (Celsius) is constant or increasing/decreasing
  • Kinetic Energy (KE) and Potential Energy (PE) are constant or increasing/decreasing

 

When referring to water, specific amounts of calories absorbed/released can be calculate at each stage.

NOTE: 1 calorie = 4.184 J

Heating curve (ENDOthermic physical change):

  • Temperature increase: solid (ice) heating up requires 0.5 calories for each increase of 1 degree Celsius for each 1 gram of water
  • Phase change: solid at 0 degrees Celsius melting to form liquid (water) at 0 degrees Celsius requires 80 calories for each 1 gram of water *NOTE: the temperature does not increase during phase changes
  • Temperature increase: liquid heating up requires 1 calorie for each increase of 1 degree Celsius for each 1 gram of water [so 1 g of liquid water increasing from the melting point (0 degrees Celsius) to the boiling point (100 degrees Celsius) requires 100 calories]
  • Phase change: liquid at 100 degrees Celsius boiling to form gas (steam) at 100 degrees Celsius requires 540 calories for each 1 gram of water *NOTE: the temperature does not increase during phase changes
  • Temperature increase: gas heating up requires 0.48 calories for each increase of 1 degrees Celsius for each 1 gram of water

Therefore, the energy REQUIRED to undergo the physical change from solid water to gaseous water is 80 + 100 + 540 = 720 calories for each 1 gram of water.

 

Cooling curve (EXOthermic physical change):

  • Temperature decrease: gas (steam) cooling down releases 0.48 calories for each decrease of 1 degrees Celsius for each 1 gram of water
  • Phase change: gas at 100 degrees Celsius condensing to form liquid (water) at 100 degrees Celsius releases 540 calories for each 1 gram of water *NOTE: the temperature does not decrease during phase changes
  • Temperature decrease: liquid cooling down releases 1 calorie for each decrease of 1 degree Celsius for each 1 gram of water [so 1 g of liquid water decreasing from the boiling point (100 degrees Celsius) to the freezing point (0 degrees Celsius) releases 100 calories]
  • Phase change: liquid at 0 degrees Celsius solidifying to form solid (ice) at 0 degrees Celsius releases 80 calories for each 1 gram of water *NOTE: the does not decrease during phase changes
  • Temperature decrease: solid (ice) cooling down releases 0.5 calories for each decrease of 1 degree Celsius for each 1 gram of water

Therefore, the energy RELEASED to undergo the physical change from gaseous water to solid water is 540 + 100 + 80 = 720 calories for each 1 gram of water

 

Use this helpful diagram of heating and cooling curves.