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Vaporization and Specific Latent Heat of Vaporization

Simple Explanation

Vaporization is the change from liquid to gas. The specific latent heat of vaporization is the energy needed to convert one kilogram of a liquid into vapour at its boiling point, with no temperature change.

Why Do We Need It?

Latent heat of vaporization is typically much larger than the heat needed for ordinary temperature changes, which explains why boiling water into steam takes so much more energy (and time, at fixed power) than heating the same water from room temperature to boiling.

Formula

Latent Heat of Vaporization

Q = mLᵥ

The heat energy needed to convert a mass of liquid into vapour (or vice versa) at a constant temperature (the boiling point), with no temperature change during the process itself.

Q
— Heat energy required, in joules
m
— Mass of the substance changing state, in kg
Lᵥ
— Specific latent heat of vaporization of the substance, in J/kg

When to use it: Use to find the heat energy needed to boil (or condense) a known mass of a substance at its boiling point.

Worked Example

Finding energy to boil water into steam

Find the heat energy needed to convert 0.5 kg of water (already at 100°C) entirely into steam (Lᵥ of water = 2,260,000 J/kg).

    Why Does This Work?

    Converting a liquid to a gas requires completely separating molecules from each other (against strong intermolecular attractive forces), which takes far more energy than simply making them vibrate or move faster within a liquid — this is why latent heat of vaporization values are typically very large compared to specific heat capacity values.

    Real-Life Example

    Why steam burns are worse than boiling-water burns

    Steam burns tend to be more severe than burns from water at the same 100°C temperature.

    As steam condenses back into liquid water on skin, it releases its large latent heat of vaporization — this releases much more energy into the skin than an equal mass of already-liquid water at 100°C would, which is why steam burns are typically more severe.

    Practice

    Find the heat energy needed to vaporize 0.2 kg of water at its boiling point (Lᵥ = 2,260,000 J/kg).

    Medium
    J

    Common mistake

    Forgetting that this formula only applies AT the boiling point — heating water from room temperature to boiling requires the specific heat capacity formula (Q=mcΔT) first, and only the final liquid-to-gas conversion at 100°C uses the latent heat of vaporization formula.

    Quick Review

    • Q = mLᵥ gives the heat needed to vaporize a mass of liquid at its boiling point.
    • Vaporization requires overcoming strong intermolecular attractions, so Lᵥ is typically very large.
    • Steam releases this large latent heat upon condensing, making steam burns especially severe.
    ← Back to Heat and Thermal PhenomenaNext: Fusion and Specific Latent Heat of Fusion →