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Why is there a hiss sound when water falls on a hot surface? I have searched a lot, asked my teachers but none of them seem to give me the logical answer to it.

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  • $\begingroup$ Related: Why is boiling water loud then quiet? $\endgroup$ – Chris Mueller Jul 16 '14 at 16:56
  • $\begingroup$ I think that one of the reason may be the creation of little spherical droplets when water touch the hot surface. If you try by yourself (it works great with hotplate as those used for camping) you will see that by dropping a very small quantity of water it will turn into a little sphere and bounce around producing an hiss sound. So maybe it's not just sudden evaporation the reason. $\endgroup$ – giulio bullsaver Sep 22 '14 at 17:46
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    $\begingroup$ I have a hypothesis but I don't have a good enough microphone to verify it. I have tried to heat up my stove and drop varying amounts of water on the stove at different temperatures - the hissing sound seems to continuously change into the little "clicks" of single bubbles for a cooler stoves and larger amounts of water. The hypothesis is the following, the hiss is just a superposition of a larger number of "clicks" and this could be seen by observing a similar structure in the frequency spectrum. $\endgroup$ – Void Sep 23 '14 at 19:46
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Due to evaporation a layer of air forms between the water droplet and the hot surface which causes the system to vibrate by letting air escape in bursts and produce sound.

I suggest reading about the description of the sound that was recorded in the Leidenfrost experiment. Article: https://www.nature.com/articles/srep00720

Interesting conclusion:

"We hypothesize that the r.m.s. amplitude of the sound reflects the area of contact the droplets have with the brass, as we expect the contact area to determine the rate of nucleate boiling."

Other interesting paragraphs:

[...] the sound produced by droplets boiling in a flat bowl machined into the same brass material [Figure 1(e)] was recorded at various temperatures [Figures 2(a) and (d)] (more details are provided in the Methods section). At 210°C, the droplet boils violently. Nucleate boiling towards the middle of the droplet creates vibrations and large movement, throwing liquid onto the surface that can sizzle.

[...]

At 225°C, the data [Figure 2(a)(iii)] shows intervals where the droplet is close to silent, with intermittent outbursts of sound. When the droplet is close to silent, the droplet is levitating above the surface, while sound is recorded when the droplet makes contact with the hot surface. This is rather random, and occasional sizzles dominate the r.m.s. amplitude in this regime. At higher temperature still, these occasional sizzles become rare and the system becomes silent, showing that the system is fully in the Leidenfrost regime.

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The reason behind the hissing sound is that the temperature of the water droplet is much lower than the hot surface. As soon as the water droplet's base touches the hot surface it quickly evaporates but still the top part of the droplet is in liquid state and there is an opposition to the water-vapour coming from below. As the water-vapour couldn't vertically go up, it starts to slide horizontally and there produces a friction between the molecules of below surface of water droplet and the molecules of water-vapour which is trying to go up, and in friction there is always sound. Hence in this case it is hissing sound.

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When it turns into a gas it is quickly turning into greater disorder. That sudden transition creates a type of white noise. Within a boiling pot that sound is re-absorbed by the surrounding water (cooling off) so you don't quite hear it the same way.

You could say that "hissing" is the universe's way of preserving the energy <-> information equations. You transform that disorder into order.

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