The Plateau Mystery
Heating curves — where the thermometer stops and the state changes
What this lesson covers
Why it matters
Heat ice steadily and log the temperature. Twice on the way to steam the thermometer just… parks. Heat pouring in, temperature refusing to move. Where do those joules GO?
The idea in plain words
The curve's five chapters. Tap the plateaus' fine print.
warm → MELT (0°C) → warm → BOIL (100°C) → warm
Why does food cook faster in a pressure cooker?
You didn't beat the plateau — you BRIBED it upward with pressure
- plateau 1: melting point — 0 °C for ice — pressure lowers it slightly; impurities (salt) lower it too (roads, freezing mixtures)
- plateau 2: boiling point — 100 °C for water — pressure RAISES it (pressure cooker); impurities raise it too
- the twist: volume oddity — ice→water CONTRACTS (ice floats!); water→steam expands ~1600×
- Higher pressure raises water's boiling point to ~120 °C
- Cooking chemistry runs faster at the hotter plateau
Predict first
During melting, the heat you keep adding goes into…
On a plateau every joule is doing DEMOLITION work on the ice lattice — none is left to speed the molecules up. That hidden pay is latent heat.
- breaking the solid's bonds — state changes, temperature doesn't — correct
- raising the temperature slowly
- the container instead
What you do
Read the full curve from ice to steam and put the right explanation on each plateau.
Check yourself
On the melting plateau, the ice-water mixture's temperature is…
Solid and liquid coexist at the melting point; the thermometer waits for the demolition to finish.
Salting icy roads works because salt…
Impurities depress the melting point — the ice finds itself above its (new) melting point and gives up.
Water is bizarre on melting: ice → water actually…
Ice's open crystal collapses into denser water — the anomaly behind floating bergs and burst pipes.
- steady at 0 °C until the LAST crystal melts — correct
- slowly rising
- briefly falling
- lowers the melting point, so ice melts below 0 °C — correct
- heats the ice chemically
- makes the road rough only
- shrinks in volume (that's why ice floats on its own melt) — correct
- expands like all solids
- keeps exactly the same volume