Specific Latent Heat Flashcards

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What is specific latent heat?

The energy needed to change the state of 1 kg of a substance without any change in temperature.

What are the two types of specific latent heat called?

Specific latent heat of fusion (solid to liquid or liquid to solid) and specific latent heat of vaporisation (liquid to gas or gas to liquid).

When do you use Lf and when do you use Lv?

Use Lf for melting or freezing. Use Lv for boiling or condensing.

What is the equation linking energy, mass, and specific latent heat?

E = m × L.

What are the units of specific latent heat, L?

Joules per kilogram (J/kg).

During melting or boiling, what happens to the temperature as energy is added?

It stays the same; the energy is used to change state.

Why does temperature not rise while a substance is changing state?

The energy is used to overcome or form forces between particles, not to make the particles move faster.

Does a substance take in or release energy when it freezes or condenses?

It releases energy to the surroundings (latent heat).

How much energy is needed to melt 0.20 kg of ice if Lf = 334,000 J/kg?

E = m × L = 0.20 × 334,000 = 66,800 J.

A 0.050 kg liquid boils when 12,000 J is supplied. What is its Lv?

L = E ÷ m = 12,000 ÷ 0.050 = 240,000 J/kg.

On a heating curve, what do the flat sections (plateaus) show?

A change of state where latent heat is taken in or given out, so temperature stays constant.

How could you find Lf for ice in a lab?

Use a heater of known power to melt ice at 0°C, time the heating, measure the mass melted, then L = (power × time) ÷ mass.

Revision Notes
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