When you’re boiling a pot of water, right before the water starts to boil if you watch carefully at the bottom of the pot there will be tiny bubbles that form and disappear. Why do they just disappear instead of floating up to the top once they’re already formed??


When you’re boiling a pot of water, right before the water starts to boil if you watch carefully at the bottom of the pot there will be tiny bubbles that form and disappear. Why do they just disappear instead of floating up to the top once they’re already formed??

In: Physics





Finally something I can answer lol.

So the heating element (or fire if you use a gas range) is obviously very hot. Much hotter than the 100 degrees C water needs to start evaporating. As you heat up the pan, that heat from the stove goes to the pan and subsequently, starts heating the water. The water at the bottom of the pan will heat up much faster than the water at the top of the pan (since it takes time for the heat to go up the water).

As the bottom of the pan gets to 100C and water starts boiling, that bubble (steam, aka water vapour and not air (though there is some small amount of it)) will start rising but immediately get in contact with more water that is not at 100C yet (further from the bottom of the pan), and ‘cool’ down and that water vapour will go back to liquid and the bubble disappear. As you keep heating it for longer, you’ll notice the bubbles get higher and higher before they disappear up until they can reach the very top and start bubbling and you’ll say it’s boiling.

You’ll also see these bubbles on the walls of the pan since the heat from the bottom of the pan can conduct through the walls of the pan and heat from the sides, and those will also be much hotter than the water itself.

Fun fact, if you want your water to boil faster, you can agitate the water as it heats up (mix with spoon or something) and it will help the water at the bottom to mix with the water at the top and transfer that heat faster.

EDIT: The laSt part about cooking was not 100% corrEct aNd therefore, I removeD it. As poiNted oUt by others, cooking has other processes coming in play such as maillarD reactions and hEat destroying pathogenic germS.

To further expand, there’s different ‘regimes’ of boiling. As the bottom surface is heated above the boiling point, we don’t actually see any bubbles until the surface is a few degrees past boiling. The larger the temperature difference, the more bubbles we see form and eventually make it to the top. This is what we refer to as nucleate boiling.

If the temperature of the surface gets too high, a stable vapor film actually forms over the surface and traps the bubbles.
This is referred to as film boiling.

[Boiling curve](https://www.engineersedge.com/heat_transfer/water_boiling_graph_curve_13825.htm)

I’m a materials engineer so here goes.

Water boiling is a phase change reaction. In any phase change there has to be nucleation of the new phase in the current phase.

Nucleation basically means small (sometimes spherical) bubbles of the second phase trapped in the first.

Since all reactions are governed by kinetics and thermodynamics both, it is possible that a reaction may be thermodynamically favorable but not kinetically.

This means, right before the water boils, it has enough energy to turn into vapor, but it does not have enough to escape the liquid

Also, there is a critical nucleation size, beyond which the second phase is stable in the first one. This is due to the different internal and surface energies of the 2 phases.

Simply, when 2 phases are in contact, they have some surface energy. Both phases also have some internal energy.

Both these energies are negative (so higher means more negative)
If the surface energy is higher than the internal energy, the nucleus will be stable and grow. OTOH, if the surface energy is lower, the nucleus will collapse.

Again, kinetics says the reaction should probably happen, since we have enough energy to grow a nucleus. But until thermodynamics comes in and says “you have crossed the critical nucleus size, you may grow further,” the nucleus will always collapse on itself

Small bubbles of water boiling – but during this time not all water is 100° C so not all water is at boiling temperature. The little bubbles of steam travel up and the water above it at a bit lower temperature cools the steam down back to water, so they don’t escape from the top.

A follow-up question on the same lines

Why then do the bubbles appear to form at specific single points at the bottom of the pan? It’s like a steady stream of bubbles from that location.

Based on the answers I went through this thread bubbles form because the vessel is heated and so adjacent water turns to vapor, but then why only specific locations?

Interestingly, if you use cold water and watch as it slowly heats up, you will see bubbles form that DO make it to the top.

Air dissolves more easily in cold water. As it heats up, the air has to come out. These bubbles will often sit on the bottom of the pot until they’re big enough and float to the top. Typically they are all gone by the time steam actually starts to develop (the disappearing bubbles).

A serious answer I think what you’re seeing are little bubbles of steam but what happens is when the steam hits the colder water above it it condenses and gives its heat to the colder water above it both waters are probably at 212 degrees boiling water is when more energy is put into the water then can be put into equilibrium so the surplus energy is given off as steam boiling water so the top temperature of water under normal conditions is 212 and then with more energy it’s steam at 212 and then at that point if there’s more energy added the water simply boils faster but that brief moment in time is when the water some of the water is below 212 and the steam transfers the steam is formed and then it transfers to heat up the surrounding water to bring it up to 212

Good question!

The bubbles you are at the bottom are actually not the same as the bubbles that you are thinking of!

The one that you’re thinking of are air bubbles, when you put a straw into the bottom water and blow air into it, the air gets pushed into the water and rises to the top.

When you boil water, some of the water at the bottom of the kettle gets so hot they turn into steam! Steam looks exactly the same as air, but with one exception – they like to give out heat and when they do, they shrink and turn back into water.

So just as the water starts to boil, these tiny stream bubbles form, but since it is surrounded by water that’s not yet at boiling temperature, they almost immediately shrink and turn back into water.

This process repeats until all water in the kettle gets to 100 degrees C, when steam can’t shrink anymore, and start rising to the top!

TLDR for the whole thread…..the bubbles (water vapor) get to cooler temps away from the heat source and the vapor reverts back to a liquid.

Oy vey

Heat *is* motion. Those little bubbles are spots where the water is so hot, and moving so fast, that it actually pushes away the water around it and becomes a little bubble of steam. The steam is water molecules zipping around bumping into the liquid water hard enough to keep it away. It’s actually lifting all of the water in the pot directly above it!

As you point out, steam is lighter than water so this bubble does start to float. As it does, it moves out of the super-hot bottom layer of water that’s right against the heating element.

As soon as the water around the bubble gets a little colder, it can’t sustain itself. The steam bumps into the cold water and loses energy. As it slows down, the water above presses in. The whole thing collapses!

Basically because the bottom of the water is the hottest, it gets to the boiling point first. Steam bubbles can only just survive there during this phase of heating. Stirring the water as it heats up will make these bubbles take longer to appear, but once they do they’ll be able to rise through to the top much faster. This is because all of the water will be heating more evenly.

I was reading an explanation, and damn it disappeared on me! WTF happened???🤨🤨