Why does water make a noise when it boils?

In general, water makes noise only when there are bubbles in it – it is actually the bubbles that make the noise. This is true for waves splashing on the seaside (90% of that sound comes from bubbles), for  fountains (the different splashing sounds come from different sizes of bubbles) and it also happens in boiling water. In the latter case, it is mainly the sound of popping bubbles that you hear.

To prove this is true, let’s look closely at a pot of water put on the stove.

As time passes, you can notice the following things happen:

1. The water starts moving, from the bottom towards the top and from the top towards the bottom. This phenomenon, called convective motion, happens because the hot water at the bottom (near the flame) is hotter and lighter…and wants to go upwards. No noise is created by this.
2. You can see bubbles forming in some specific areas at the bottom of the pot (normally, closer to places where the surface is not flat). Those bubbles (of vapour) rise a bit and disappear into the liquid. This happens because, as they go upwards propelled by buoyancy, they encounter water at a lower temperature….which turns the vapour back into liquid. The liquid around fills the gap and this creates a popping noise….which is what we hear. This also creates some small vibrations.
3. As more and bigger bubbles are formed, the typical boiling sound is heard and the pot is also vibrating.
4. If you add salt to the water at this point, the bubbles (and the sound) disappear for a few seconds. This is because adding salt increases the boiling point of water, therefore the temperature is still the same but no bubbles are formed. And no bubbles, no noise!
5. If you wait longer (without adding the salt), you might experience another phenomenon: the noise disappears by itself. This happens because the amount of vapour produced is so high that no more bubbles pop in the liquid…they pop at the surface. And this is a much gentler sound.