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Treebeard's Stumper Answer
28 September 2001

The Sound of Boiling Water

Put a kettle of water on the kitchen stove and listen to the sound of boiling water. That peaceful sound tells a story. It starts low and grows louder and harsher as the water heats up. Then the sound becomes softer just before your water starts to boil and make steam. Why is there that brief moment of "calm before the steam" that lets you know your water is ready even before the whistle? What about boiling water in space on the International Space Station (ISS) with a normal atmosphere but no force of gravity to distinguish between up and down?

That boiling kettle hides real mysteries. What is boiling? It's not just the state change of liquid water to vapor. That happens all the time as water evaporates, with no sound at all. Boiling is something else. Why does boiling water make any sound at all? Take the lid off and peer inside (with eye protection!) to discover the details.

I made a six minute MP3 recording (750K) of the sound of boiling water from start to finish. It's a simple mono recording encoded as the lowest quality mp3, but you can hear all the details. The background hiss is the gas in my kitchen stove. Listen to those interesting growls and rhythms!

The "calm before the steam" is subtle, from around 4:15 to 4:40 in the mp3. If you listen with Winamp, you can get a realtime oscilloscope or spectrum analysis. Richard Horne's Spectrogram program is another useful tool.

Recording the sound of boiling water...

The sound of boiling water is made by the bubbles. Water quietly turns to vapor all the time as it evaporates. Over a flame, the hot water at the bottom forms vapor bubbles that rise into cooler water and collapse, making a sound by cavitation. In time the bubbles make it to the surface without all that noisy popping, and that's why boiling has a softer sound. There's no convection or buoyancy in microgravity, so everything is different. Recent NASA studies on the ISS show that a single large vapor bubble forms near the heat source with little movement.


When we boil water, we expect it to go through a phase change and become a vapor or gas. But it doesn't happen all at once. Small bubbles form on the bottom of the kettle close to the heat source. They grow and finally detach to rise by buoyancy because they are less dense than the surrounding water. As the vapor bubbles rise into cooler water, they recondense into liquid and finally pop, making a noise. Warm water also rises by convection along with the bubbles, and the cooler surface water sinks, so the overall temperature of the water rises. We hear the bubble activity increase until the water is hot enough that the bubbles make it to the surface without popping. That's when the sound becomes softer, and that's what a "rolling boil" really is. If there's a lid on the kettle, it takes a moment to build up enough pressure to force the steam out the spout then which increases the sound again.

I used Cool Edit Pro to sample my cassette tape recording of the sound of boiling water. Then I used Richard Horne's Spectrogram program to do a spectrum analysis. Here I've lined up the waveform with it's spectrum in time. The "calm before the steam" is the yellow part of the waveform, beginning just after 4:20.

I was expecting something more dramatic. I notice a slightly darker blue in the middle frequencies between 1-4 KHz. I can hear the difference better than I can see it in these charts. The classic Francis Ford Coppola movie The Conversation is about how hard it is to do audio signal processing.

Boiling isn't just water turning to steam. It's an elaborate mechanism of heat transfer through the water in the kettle from bottom to top. Even gasoline and alcohol and other solvents boil as the liquids burn on the surface. Boiling is an important process in many engineering systems like refrigerators, air conditioners, power plants, cars, and spacecraft cooling systems.

Normal boiling depends on buoyancy and convection. Heat rises. But there's no up and down in the microgravity of the Space Shuttle and the International Space Station. That's why NASA is interested in boiling.

The Science@NASA site gives this explanation:
When a pool of liquid is heated on Earth, gravity causes hotter regions in the liquid to rise, and cooler, more dense parts to sink -- a process called "convection." This motion spreads the heat around inside the liquid. Once it begins to boil, buoyancy sends bubbles hurling upward, creating a "rolling boil."

All of this motion within the liquid makes the physics of the situation much more complex.

Without convection or buoyancy, the process unfolds differently. Heated fluid doesn't rise, and instead just sits next to the heater surface and continues to get warmer. Regions of liquid away from the heater remain relatively cool. Because a smaller volume of water is being heated, it comes to a boil much more quickly. As bubbles of vapor form, though, they don't shoot to the surface -- they coalesce into a giant bubble that wobbles around within the liquid.

Boiling is really simpler in space without convection and buoyancy, just like Newton's laws of motion are easier to understand in space without friction and gravity. What about a candle flame on the ISS with an atmosphere but no gravity? What about opening a beer or soda? Think before you click on those links! What other ordinary phenomena are interestingly different in the microgravity of space?

Closer to home, this answers another stumper I thought of while rafting the Green River through Desolation and Gray Canyons last summer in Utah. That section of the river is mostly flat water punctuated by rapids. You can always hear the roar of the approaching whitewater rapids around the next bend. Why are the river rapids so noisy, even the small ones with no great rocks or falls? I'm sure it's the bubbles!

Here are some links for further research:

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