Changing a substance’s state takes energy ā but unlike heating it up, none of that energy shows up as a temperature change. Latent heat is what that “hidden” energy is called.
š What you need to know
During a phase change, thermal energy is transferred without changing the substance’s temperature
This thermal energy is calculated using Q = mL
Specific latent heat is the energy required to change the state of 1 kg of a substance without changing its temperature
There are two types: fusion (solid ā liquid) and vaporisation (liquid ā gas)
For any given substance, the specific latent heat of vaporisation is always greater than the specific latent heat of fusion
The Specific Latent Heat Equation
Thermal energy during a phase changeQ = mL
Where Q is heat energy transferred in joules, m is mass in kilograms, and L is the specific latent heat of the substance in J kgā»Ā¹. Rearranged, L = Qām, which is exactly the definition above written as an equation.
Fusion vs Vaporisation
Specific latent heat of fusion applies to melting and freezing ā the energy released when 1 kg of liquid freezes, or absorbed when 1 kg of solid melts, at constant temperature.
Specific latent heat of vaporisation applies to boiling and condensing ā the energy released when 1 kg of gas condenses, or absorbed when 1 kg of liquid vaporises, at constant temperature.
Vaporisation always needs more energy than fusion for the same substance. Melting only needs to partially overcome the intermolecular forces holding a solid together, loosening the structure enough to flow. Vaporisation needs to completely overcome those forces, separating particles enough to become an independent gas ā a much bigger job.
Vaporisation always requires more energy per kilogram than fusion, because it fully separates particles rather than just loosening them
Heating Curves: Seeing Latent Heat in Action
A heating curve shows how a substance’s temperature changes as thermal energy is supplied at a constant rate. It has two kinds of sections: sloped sections, where the substance is heating up and its kinetic energy is rising, and flat sections, where a phase change is under way and the energy is going entirely into potential energy instead.
Sloped sections show rising temperature; flat sections show a phase change happening at constant temperature
Quick recap: Q = mL. Fusion applies to melting/freezing; vaporisation applies to boiling/condensing. Vaporisation always needs more energy per kilogram than fusion.
WE 1
Determine the energy needed to melt 350 g of a solid with a specific latent heat of fusion of 1.8 Ć 10āµ J kgā»Ā¹.
Step 1 ā Identify the correct latent heat
Melting is a solid-to-liquid change, so fusion applies
Step 2 ā Substitute into Q = mLQ = 0.350 Ć (1.8 Ć 10āµ)= 63 000 J = 63 kJ
WE 2
A heater rated at 1800 W supplies energy at a constant rate to vaporise 750 g of a liquid with a specific latent heat of vaporisation of 2.0 Ć 10ā¶ J kgā»Ā¹. Ignoring energy losses, determine the time taken to fully vaporise the liquid, in minutes.
Step 1 ā Find the energy requiredQ = mL = 0.750 Ć (2.0 Ć 10ā¶) = 1.5 Ć 10ā¶ JStep 2 ā Use P = Q/t, rearranged for tt = Q Ć· P = (1.5 Ć 10ā¶) Ć· 1800ā 833 s ā 13.9 minutes
š” Top tips
Match the type of latent heat to the phase change described ā fusion for melting/freezing, vaporisation for boiling/condensing
Mass must be in kilograms before substituting into Q = mL
Remember specific latent heat of vaporisation is always larger than fusion for the same substance ā a useful sanity check on your answer
On a heating curve, a flat section always means a phase change, never a stalled heater
ā Common mistakes
Using the specific latent heat of fusion for a boiling or condensing problem, or vice versa
Forgetting to convert mass from grams to kilograms before calculating Q
Assuming melting and boiling require the same amount of energy per kilogram
Interpreting a flat section of a heating curve as “no energy transfer,” rather than energy going entirely into potential energy
Up next: Thermal Conduction ā where we look at how thermal energy actually moves through a solid.
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