• Question: I notice when I boil a pot of water, it makes a hissing sound, and after about 5 minutes it almost stops, then it starts up again progressively until it starts to boil. Why does it get quiet and start again?

    Asked by nicenadia to Alessandro, Angela, Claudia, Marina, Phil on 12 Mar 2013.
    • Photo: Phillip Wilkinson

      Phillip Wilkinson answered on 12 Mar 2013:

      The hissing sound is the transfer of heat into the water molecules. As the atomic bonds are broken and gas is released into the air it produces sound. This sound is then absorbed by the water and makes that ‘hissing’ sound.

      As for why it stops and starts again, I don’t know. 🙁

    • Photo: Claudia Krehl

      Claudia Krehl answered on 12 Mar 2013:

      I will have to listen the next time I boil water!

    • Photo: Alessandro Guazzi

      Alessandro Guazzi answered on 12 Mar 2013:

      I think that what might be happening is that the first noise is the air that is trapped in the water, that is escaping towards the top as the water heats up. This has nothing to do with the water which is actually boiling other than it was trapped there to begin with and the new energy acquired from the heating is enough to allow it to escape. Then, as most of the trapped air has gone, the water becomes quiet again, until you finally start to create bubbles of vapour as the water becomes warmer and warmer, which are the start of the renewed noise.

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