The phrase ‘a watched kettle never boils’, or a watched pot as Americans might prefer say, is a familiar phrase. We have probably all stood waiting for water boil thinking it is taking a long time. This might be in part because the rate of boiling does indeed slow down during the heating process and then speed up towards the end.
When an electric kettle is first switched on the element in the bottom of the kettle heats up causing heat to be transferred by conduction to the water. The water adjacent to the element rises in temperature becomes less dense, moves towards the surface and transfers heat by natural convection to the contents of the kettle. As the temperature of the water rises, tiny bubbles form on the element due to local boiling. Bubbles are dislodged by new ones forming and float up to the surface giving the appearance that complete boiling is imminent. However, as the temperature rises further the element becomes completely covered by a film of vapour that insulates the element from the water and slows down heat transfer to the water. This delays boiling until the element has pumped enough energy (heat) into this film for heat transfer to occur across it from the element to the water. Sections of the film tend to break away and belch onto the surface of the water. This process of large bubble formation and belching on the surface usually establishes itself fairly quickly once the first one has broken free and we see the familiar violent boiling of the kettle.
So the watched kettle has boiled but only after what might have seem like an interminable delay. If you have a transparent electric kettle then you can watch this happen, otherwise you could watch a YouTube video – possible the most boring video on YouTube?
The process described above is known as the Liedenfrost effect and is illustrated graphically in the chart below, which is based on Figure 6.16 in ‘The Design and Simulation of Thermal Systems‘ by NV Suryanarayana and Oner Arici published by McGraw-Hill. There are a number of more comprehensive explanations available, for example by Jearl Walker. The Leidenfrost effect is also responsible for the way water disperses in liquid droplets across a very hot surface instead of evaporating as steam, see this Youtube clip for more explanation.