Savchenko Problem 5.10.3
Physics · optics
Problem
Why does water in a vessel covered with a lid boil faster than in an open one?Intuitive explanation
The key is understanding how vapor pressure and equilibrium work. When the lid traps steam above the water, that vapor can't escape—it builds up until it reaches saturation, raising the pressure above the liquid surface. Higher pressure lowers the boiling point (the temperature at which vapor pressure equals atmospheric pressure), so the water reaches its new, lower boiling point faster. In an open vessel, steam constantly escapes, pressure stays at 1 atm, and you must heat to the full 100°C.
Intuitive explanation
The key is understanding how vapor pressure and equilibrium work. When the lid traps steam above the water, that vapor can't escape—it builds up until it reaches saturation, raising the pressure above the liquid surface. Higher pressure lowers the boiling point (the temperature at which vapor pressure equals atmospheric pressure), so the water reaches its new, lower boiling point faster. In an open vessel, steam constantly escapes, pressure stays at 1 atm, and you must heat to the full 100°C.