Melting, Boiling and Evaporation

When a substance melts or boils, the temperature stays constant even though energy is still going in: the energy is used to break the forces between particles, not to speed them up. Evaporation happens at any temperature, from the surface only; the fastest particles escape, so the liquid left behind cools.

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Section 1 of 14

Learning objectives

Name the changes of state and know water's melting and boiling points.
Explain why the temperature stays constant during melting and boiling.
Interpret heating and cooling curves.
Explain evaporation and cooling, and compare evaporation with boiling.

Pause and think

You put a thermometer in a pan of boiling water and turn the heat up. Does the reading go above $100$ °C?

Section 2 of 14

Core idea

Start hereA pan of boiling water stays at 100 °C

Turn the hob up under boiling water and it boils faster, but the thermometer stays at 100 °C. Where's the extra energy going? Into pulling molecules apart, not speeding them up.

The big idea

During a change of state, temperature is constant; energy breaks bonds. $E=mL$ , $L$ = specific latent heat.

1. Heating curves

Heating curve of ice to steam

The flat sections are changes of state. Boiling takes much longer than melting, because pulling particles completely apart needs far more energy.

Sloping parts: temperature rising ( $mc\Delta\theta$ ).
Flat parts: change of state ( $mL$ ).
Boiling plateau is longer: $L_v>L_f$ .

E=mL

$L_f$: latent heat of fusion (melting), J/kg.
$L_v$: latent heat of vaporisation (boiling), J/kg.

Worked exampleIce to warm water

0.5 kg ice at 0 °C to water at 20 °C ( $L_f=3.3\times10^5$ J/kg, $c=4200$ ).

Goal: Total energy.
Strategy: Two stages: melt, then warm.

$0.5\times3.3\times10^5=165000\text{ J}$
Melt at 0 °C.
$0.5\times4200\times20=42000\text{ J}$
Warm the water.
$207000\text{ J}$
Add.

Answer

207

kJ.

✓ CheckMelting takes about 4× the warming: ice keeps drinks cold ✓.

Common misconceptionOne formula only State changes need

mL

and

mc\Delta\theta

for each temperature change.

2. Evaporation

Evaporation: only the fastest particles escape

Those that escape carry away more than their share of energy, so the liquid cools.

Fastest particles escape; average energy of those left falls: cooling.
Faster with higher temperature, larger surface, wind.
Boiling: throughout the liquid at a fixed temperature.

Pause and think

Why does sweating cool you?

Summary

Flat = state change.
$E=mL$ .
Evaporation cools.

Latent heat: Energy for a change of state without temperature change.

HistoryThe ice that didn't warm. Joseph Black noticed in the

1760

s that melting ice stayed at

0

°C while absorbing lots of heat. He called this hidden heat "latent heat", from the Latin for "hidden". His friend James Watt used the idea to make steam engines far more efficient.

Fun factClay pot fridges. In hot, dry regions, a "pot-in-pot" fridge keeps vegetables fresh without electricity: a clay pot sits inside a bigger one with wet sand between them. As water evaporates from the outer pot, it cools the inside by up to

15

°C.

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