WHY IS BOILING WATER LOUD ?

A lot of the times we see water in electric kettle makes the most noise sixty to ninety seconds before the water comes to a full boil. We have been fooled many times by the noisy kettle, only to discover that the water was not yet hot enough for tea. The kettle is only at a full boil after the noise has subsided.

We have noticed the same phenomenon with many other kettles, including conventional kettles on kitchen ranges; it is not a peculiarity of this electric kettle.

Why does the boiling become quieter as the water reaches full boil?



ANSWER :- 

There are three phenomena that occur before vigorous boiling of water that produce sound.

1) Air dissolved in water on heating forms small air bubbles at the bottom of the container. These air bubbles get released from the bottom of the container on reaching a sufficient size. The process of release produces a sound of frequency ~ 100Hz.

2) On boiling, small vapor bubbles get produced at the bottom of the container and also produce sound of ~ 100 Hz on release. However, they cool down before they reach the surface of water and collapse. This collapsing produces a sound of frequency ~ 1 KHz.

3) Collapsing vapor bubbles agitate the water to release small micro air bubbles from water and also from the air trapped in the vapor bubble. This production of micro air bubbles produces a sound of ~ 35 - 60 kHz.