Apply latent heat without treating a phase change as a temperature rise.
Method and worked examples
During a pure substance’s phase change at constant pressure, energy changes particle arrangement while temperature remains approximately constant. Melting and vaporisation absorb energy; freezing and condensation release it. Sublimation is solid to gas and deposition is gas to solid. Evaporation occurs at a liquid surface below boiling; boiling occurs throughout the liquid at its boiling condition.
Use Q=mL for a complete phase change, with L in kgJ. Choose latent heat of fusion for melting or freezing and latent heat of vaporisation for boiling or condensation. Give a positive magnitude unless a sign convention is requested. For a process that also warms the material, calculate mcΔT and mL stages separately, then add them.
Example 1
Melt 0.10kg of ice already at 0∘C. Use Lf=kg3.34×105J and neglect losses.
Only a phase change is needed: Q=mLf=0.10×3.34×105=33400J.
Example 2
Condense 0.020kg of steam at its boiling temperature. Use Lv=kg2.26×106J. Find released energy magnitude.
Qreleased =mLv=0.020×2.26×106=45200J. The energy leaves the steam.
Watch out:Energy is still transferred to separate particles during vaporisation rather than increase the temperature.
15. Change of state and latent heat | Year 11 Physics | Ascent Arcade