Melting and boiling
Heat keeps flowing in — so why doesn't the temperature rise?
Ice is a crystalline solid and melts at 0°C; a boiling liquid stays at its boiling point. In both stages heat is absorbed continuously, yet the temperature does not change — the absorbed heat goes into changing the state of matter. The boiling point of water falls as the air pressure drops, so the higher the altitude, the lower the boiling point.
Temperature vs. heating time
Two flat segments: melting at 0°C and boiling at the boiling point, absorbing heat at constant temperature. Horizontal axis: physical time.
Think about it
What is the boiling point of water on a plateau at an altitude of 4 km? Why do people there cook with pressure cookers?
About 87°C. The higher the altitude, the lower the air pressure and the lower the boiling point of water, so food is hard to cook through; a pressure cooker raises the pressure inside the pot, which raises the boiling point. Try setting the altitude to 4 km and watch the height of the boiling plateau on the curve.
Model assumptionsStarting from ice at −20°C · constant power, no heat loss · boiling point estimated from pressure at altitude · clock sped up 20×
Data log
Set the parameters and click “Record data”. Record several rows with different parameters to plot their relationship and fit a straight line; pause and record at different times to study how a reading changes with time.