Mistake Master
The energy of phase changes
Heat a beaker of ice steadily and the thermometer does something strange: it stalls at the melting point, climbs, stalls again at boiling. Those flat stretches are where energy is spent pulling molecules apart, not speeding them up.
§1
Sloped warming and flat phase changes.
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A heating curve plots temperature against energy added. On the sloped sections, a single phase is warming and the temperature rises. On the flat plateaus, a phase change is happening.
During a plateau the temperature stays constant, even though energy is still being added, because that energy goes into breaking intermolecular forces (separating the particles) rather than speeding them up.
The two phase changes are not equal: vaporization (boiling) requires more energy than fusion (melting), because boiling must fully separate the molecules against their intermolecular attractions. So the boiling plateau is longer.
§2
Reading a heating curve.
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Tell warming apart from phase change.
- Identify sloped sections. Rising temperature means a single phase is warming (energy raises kinetic energy).
- Identify plateaus. Flat regions are phase changes; temperature is constant while IMFs are overcome.
- Assign the plateaus. The lower-temperature plateau is melting (fusion); the higher one is boiling (vaporization).
- Compare plateau lengths. The boiling plateau is longer, because vaporization needs more energy than melting.
§3
The pieces you'll meet.
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Warming versus phase change.
§4
Worked example: reading the curve for water.
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Curve. Ice warms (sloped) to 0 °C, plateaus while melting, warms as liquid to 100 °C, plateaus while boiling, then warms as steam.
Plateaus. Temperature is constant at 0 °C (melting) and 100 °C (boiling) even as heat is added, because the energy breaks intermolecular forces.
Which is longer? The boiling plateau is much longer than the melting plateau, because vaporizing water takes far more energy than melting it.
Key point. During a plateau, adding heat does not raise the temperature — it changes the phase by separating the molecules.
§5
Mistakes that cost real points.
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"On a heating-curve plateau, nothing is happening because the temperature isn't changing."
A lot is happening: energy is being added and used to break intermolecular forces, changing the phase. The temperature is flat precisely because the energy goes into separating particles, not speeding them up.
Fix. Read a plateau as an active phase change: energy in, IMFs breaking, temperature constant.
"Melting and boiling require the same amount of energy."
Vaporization (boiling) requires more energy than fusion (melting), because boiling fully separates the molecules against their attractions. That is why the boiling plateau on a heating curve is longer than the melting plateau.
Fix. Expect the boiling plateau to be longer; vaporization costs more energy than melting.
"During a phase change, added heat raises the temperature."
During a phase change the temperature stays constant; the added heat goes into breaking intermolecular forces, not increasing kinetic energy. Only on the sloped (single-phase) sections does added heat raise the temperature.
Fix. Attribute temperature rise to the sloped sections; during a plateau, heat changes phase at constant temperature.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.