Phase Changes & Phase Diagrams

Why boiling water stays at 100 C, where the heat 'disappears' to, and how one map charts every state of a substance.

High schoolIntro Gen ChemUni Year 1
⏱️ About 18 min

Put a pot of water on to boil and watch a thermometer in it. The temperature climbs steadily to 100 C — and then stops dead, even though the burner is still roaring. Where is all that energy going, if not into raising the temperature? The answer reveals what a phase change really is.

💡
The big idea: Adding heat to a substance does one of two things: it raises the temperature, OR it drives a change of state at constant temperature. During melting and boiling, the energy goes into pulling particles apart, not into speeding them up — which is why the temperature pauses.
🎯 By the end, you'll be able to
  • Name the phase changes (melting, freezing, vaporisation, condensation, sublimation)
  • Read a heating curve and explain its flat plateaus
  • Explain latent heat: energy that changes state at constant temperature
  • Read a phase diagram, including the triple point and critical point
📎 Helpful to know first

The changes of state

Matter shifts between solid, liquid and gas as you add or remove energy. Each transition has a name:

  • Melting (solid → liquid) and freezing (liquid → solid)
  • Vaporisation (liquid → gas) and condensation (gas → liquid)
  • Sublimation (solid → gas directly) and deposition (gas → solid directly)

Which way you go depends on whether you add heat (toward gas) or remove it (toward solid). What holds particles together in the first place is their intermolecular forces — stronger forces mean higher melting and boiling points.

The heating curve: steps and plateaus

Heat a block of ice steadily and plot temperature against energy added. You don't get a straight ramp. You get a staircase: sloped sections where the temperature rises, separated by flat plateaus where it doesn't move at all.

The sloped parts are the ice, then water, then steam simply warming up. The plateaus are the phase changes: melting (at 0 C) and boiling (at 100 C for water at 1 atm). During a plateau, every joule of energy goes into breaking particles apart, not into making them move faster — so the temperature holds steady.

🔑 Latent heat: the hidden energy of a phase change
Latent heat is the energy absorbed or released during a phase change at constant temperature. Melting a solid needs the heat of fusion; boiling a liquid needs the heat of vaporisation. For water, vaporisation takes far more energy than melting — which is why a steam burn is so much worse than one from hot water: the steam dumps all that latent heat into your skin as it condenses.
\[ q = m \cdot L \]
Heat for a phase change: q = mass * latent heat (L). Use the heat of fusion for melting/freezing, or the heat of vaporisation for boiling/condensing. Temperature does not change during this step.
⚠️ Two things people get backwards
  • Temperature is constant during a phase change. While ice melts, both the ice and the water sit at 0 C. The heat isn't 'missing' — it's going into breaking bonds, not raising temperature.
  • Boiling is not the same as evaporation. Evaporation happens only at the surface, at any temperature. Boiling happens throughout the liquid once its vapour pressure equals the surrounding pressure.
📝 Worked example: How much heat is needed to boil away 10.0 g of water already at 100 C? (Heat of vaporisation of water = 2260 J/g.)
  1. The water is already at its boiling point, so this is a pure phase change: q = m × L.
  2. q = 10.0 g × 2260 J/g.
  3. = 22 600 J = 22.6 kJ — all of it going into separating the molecules into vapour, at a steady 100 C.
✓ 22 600 J (22.6 kJ).

The phase diagram: a map of states

A phase diagram plots pressure (vertical) against temperature (horizontal) and shades in which state is stable under each combination. Lines divide the regions; crossing a line is a phase change. Two special points stand out.

✨ Triple point and critical point
The triple point is the single pressure-and-temperature at which solid, liquid and gas all coexist in equilibrium. The critical point is where the liquid-gas boundary simply ends: beyond it, at high enough temperature and pressure, liquid and gas become indistinguishable — a supercritical fluid with properties of both.

Why water is the weird one

On most phase diagrams the solid-liquid line tilts slightly to the right (more pressure favours the solid). Water's tilts to the left: increase the pressure on ice and it can melt. That is because ice is less dense than liquid water — the reason ice floats, and a quirk that keeps lakes liquid beneath a frozen surface.

✏️ Practice: How much heat is needed to melt 50.0 g of ice at 0 C? (Heat of fusion of water = 334 J/g.) Give your answer in kilojoules (kJ).
kJ
Solution
  1. The ice is at its melting point, so this is a phase change: q = m × L.
  2. q = 50.0 g × 334 J/g = 16 700 J.
  3. Convert to kilojoules: 16 700 J ÷ 1000 = 16.7 kJ. The temperature stays at 0 C throughout.

Check your understanding

1. While a pot of pure water is actively boiling at 1 atm, what happens to its temperature as you keep heating?
During boiling the added energy goes into vaporising the water (latent heat), not into raising temperature. The liquid stays at 100 C until it has all turned to steam.
2. During melting, where does the added heat energy actually go?
Latent heat of fusion goes into overcoming the intermolecular forces that hold the solid together. Temperature stays constant until melting is complete.
3. What is the triple point on a phase diagram?
The triple point is the unique pressure and temperature at which all three phases coexist in equilibrium. The critical point, by contrast, is where the liquid-gas distinction disappears.
🔎 Related lessons
Previous: The Ideal Gas Law Next: Solutions & Colligative Properties Intermolecular Forces
✅ Key takeaways
  • Phase changes: melting/freezing, vaporisation/condensation, sublimation/deposition.
  • A heating curve has sloped sections (warming) and flat plateaus (phase changes).
  • Latent heat changes state at CONSTANT temperature: q = m*L (fusion or vaporisation).
  • A phase diagram maps state versus pressure and temperature; lines are phase changes.
  • Triple point = all three phases coexist; critical point = liquid and gas merge.
➡️ Phase changes are about a pure substance. But dissolve something in a liquid and its freezing and boiling points shift in predictable ways. Next: how solutes bend these rules through colligative properties.
Want to test yourself on this? Try the Chemistry practice test →