Solution

Statement

$5.10.3.$ Why does water boil faster in a vessel closed with a lid than in an open one?

Solution

In an open vessel, continuous evaporation of water occurs from the surface. Since the specific heat of vaporization of water is very high, this process takes away a significant amount of thermal energy from the liquid. In addition, convective currents constantly occur above the open surface: hot vapor rises, and cold room air takes its place, additionally cooling the water.

In a vessel closed with a lid, two important changes occur:

1. The space under the lid quickly fills with saturated vapor. **Dynamic equilibrium** is established between the liquid and the vapor (the number of molecules leaving the liquid equals the number returning). The net heat loss due to the phase transition ceases.
2. The lid prevents convective heat exchange with the environment, trapping a layer of hot vapor that acts as a thermal insulator.

As a result, all the energy supplied by the heater goes solely into heating the water itself, allowing it to reach the boiling point faster.

*(Important note: it is assumed that the lid is not hermetically sealed, and the pressure above the water remains equal to atmospheric pressure. If it were hermetically sealed, the pressure inside the vessel would increase, which would raise the boiling point, and the process could potentially take longer).*