Request a grown-up to fill a vessel with water, cover it and heat it on the stove. (An old-fashioned kettle will work just as well!) Now observe what happens.
After sometime you will hear a hissing sound. The hissing sound becomes louder and louder and finally stops.
What’s the reason for the hissing?
When the kettle is kept on the fire, the bottom layer of water gets heated first, becomes lighter and rises up in the form of tiny bubbles. The bubbles meet colder water on the way up and burst.
It is the bursting of bubbles that gives rise to the hissing sound. As the temperature rises, a larger number of bubbles travel to the surface and burst and so the hissing becomes louder. When the water is uniformly hot, the bubbles will not burst. The hissing stops and of course, the water starts boiling.
Picture Credit : Google
