Changes of State and Specific Latent Heat

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CognitoChanges of state and latent heat

Key facts & equations

Changes of state and heating curves

Changes of state: solid → liquid (melting), liquid → gas (evaporating/boiling), and reverses.
During a change of state, temperature stays constant even though energy is being supplied.
The energy goes to breaking intermolecular forces (not increasing KE).
Heating curve: flat sections at melting point and boiling point.
Steeper gradient = faster temperature rise = smaller specific heat capacity or less mass.

Latent heat

Specific latent heat: energy needed to change the state of 1 kg of substance without temperature change.
E = mL. E = energy (J), m = mass (kg), L = specific latent heat (J/kg).
Specific latent heat of fusion: solid → liquid.
Specific latent heat of vaporisation: liquid → gas (much larger than fusion — more bonds broken).
Water: L_fusion = 334,000 J/kg. L_vaporisation = 2,260,000 J/kg.

Internal energy

Internal energy: total kinetic and potential energy of all particles in a substance.
Heating increases internal energy: either increases temperature (KE) or causes state change (PE).
Temperature reflects average KE of particles. State change increases PE (not KE).
Solid: strong forces, low PE. Gas: forces broken, high PE.
Internal energy = total KE + total PE of particles.

Particle model of changes of state

Melting: energy breaks forces between particles in solid lattice → particles can move past each other.
Boiling: energy breaks all intermolecular forces → particles escape to gas phase.
Condensation: gas particles slow down, forces reform → liquid.
Evaporation (below boiling point): faster particles at surface escape → cooling effect.
Sweating cools: evaporation of water removes latent heat from skin.

Exam questions — 4 questions · 11 marks · AQA 8463 style

3 marksLatent heat calculationAQA 8463 P3 style 🔢 Calculator

Calculate the energy needed to melt 2 kg of ice at 0°C. (Specific latent heat of fusion of water = 334,000 J/kg) (2 marks)

Hint: E = mL. Latent heat of fusion is given. Multiply mass by latent heat.

+30 XP

1 markFlat section on heating curveAQA 8463 style

Why does temperature stay constant during melting even though heat is being supplied?

A The energy is being lost to the surroundings

B The energy increases the kinetic energy of particles so they slow down

C The energy is used to break intermolecular forces between particles, not to increase their kinetic energy

D Melting is an exothermic process that releases energy

4 marksCompare latent heatsAQA 8463 style

The specific latent heat of vaporisation of water (2,260,000 J/kg) is much larger than the specific latent heat of fusion (334,000 J/kg). Explain why. (3 marks)

Hint: Compare what happens to intermolecular forces during each change: melting = partially broken (particles still close). Boiling = fully broken (particles far apart).

+40 XP

1 markEvaporation coolingAQA 8463 style

Why does sweating cool the body?

A Water from sweat dissolves heat from the skin

B Sweat conducts electricity which removes heat

C Faster-moving water molecules escape (evaporate), removing latent heat from the skin surface

D Sweat is colder than the body so it absorbs body heat

Changes of State and Specific Latent Heat

Changes of state and heating curves

Latent heat

Internal energy

Particle model of changes of state

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