I Expected visible vapor time of boiling water at STP without any heat?

[shakhfenix]

TL;DR Summary

Boiled 400ml water, turned off heat. Visible steam kept rising for 30+ minutes. At ~60°C, it was still ongoing. Classical thermodynamics doesn’t explain this. What’s the expected vapor time without heat?

While boiling water in a standard stainless steel milk jug (open top, approx. 10 cm diameter), I happened to notice two intriguing phenomena under simple and reproducible conditions.
• Approx. 400 ml of filtered water was used.
• Heat was applied via direct flame until a continuous bubbling boil was reached.
• The environment was calm and draft-free, windows closed, ambient temperature stable.
• The jug was not covered, and no lid or insulation was used.
• I filmed everything in time-lapse mode (1 frame every 2 seconds), using a fixed tripod and natural lighting.
• The term “visible vapor” refers specifically to the white condensation cloud, not to invisible water vapor.

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First, I was surprised at how long it took for the water to stop visibly steaming after the heat was turned off.

Then, I found it even stranger that when I briefly turned the heat back on, the visible vapor quickly vanished, instead of increasing.

To better understand what I was seeing, I decided to frame a very basic experiment:
1. I heated the water to a full boil.
2. I turned off the heat and timed the persistence of visible vapor using the time-lapse footage.
3. Later, I turned the heat back on for a short time, then turned it off again.

The entire experiment took less than 40 minutes. There were no additions to the water (no coffee, sugar, salt, etc.) — just pure boiling water.

Since I am not a physicist, I asked AI models, including ChatGPT, to explain the expected behavior of steam in such a setup.

That’s when things became interesting.

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ChatGPT (in Deep research mode) produced the following [AI chatbot section redacted by the Mentors (not a valid reference)]

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So here’s the key question:
According to classical thermodynamics, how long should visible vapor persist after turning off the heat under these controlled conditions?
And if reapplying heat briefly causes the vapor to stop — why?

I’m not asking for explanations of what I observed. I’m asking:
What would be the expected behavior in theory?

[tik tok “reference” deleted]

 

[Dale]

I have seen visible clouds coming off a calm pond on a winter’s morning for 40 minutes at much lower temperatures. I am not sure why you think this is something problematic in any way.

 

[DaveE]

You are also heating the air above (and around your pot). The vapor pressure of water is very temperature dependent. So when it vaporizes it does so into warm air, but as it travels away (rises) it encounters cooler air which can't hold as much water vapor and some of it condenses. This is about the changing temperature of saturated (high humidity) air, not so much about boiling water.

As @Dale said, this also happens at lower temperatures. What you are seeing is clouds (or fog), just like to ones floating in the air above your house; no boiling required, just too much water vapor and lowering temperature.

Note that pressure matters too, but let's leave that out for now. Your kitchen is essentially constant pressure.

 

[sophiecentaur]

TL;DR Summary: Boiled 400ml water, turned off heat. Visible steam kept rising for 30+ minutes. At ~60°C, it was still ongoing. Classical thermodynamics doesn’t explain this. What’s the expected vapor time without heat?

how long it took for the water to stop visibly steaming

If you remember that steam is an invisible gas. What you see is not steam so you should not be surprised that it doesn't behave like steam.

 

[shakhfenix]

You are also heating the air above (and around your pot). The vapor pressure of water is very temperature dependent. So when it vaporizes it does so into warm air, but as it travels away (rises) it encounters cooler air which can't hold as much water vapor and some of it condenses. This is about the changing temperature of saturated (high humidity) air, not so much about boiling water.

As @Dale said, this also happens at lower temperatures. What you are seeing is clouds (or fog), just like to ones floating in the air above your house; no boiling required, just too much water vapor and lowering temperature.

Note that pressure matters too, but let's leave that out for now. Your kitchen is essentially constant pressure.

View attachment 359453

Thank you both for the contributions. I’d like to respectfully steer this thread back to what I originally asked — not observational anecdotes or general explanations of condensation, but what classical thermodynamics actually predicts.

It’s true that vapor can condense and appear as “fog” under various ambient conditions. But thermodynamic textbooks define visible vapor as a phenomenon occurring when water is close to boiling, and fading as the temperature decreases — not as something that remains stable for 30+ minutes, and certainly not something that disappears when heat is reapplied and returns when the temperature drops again.

What makes this observation challenging is not just that it happens — but that it contradicts the accepted explanation:

• When I reapply heat, the vapor disappears (even as temperature increases).
• When I turn it off again, the vapor returns (while temperature is dropping).

According to the literature, this behavior should not occur.

If we keep accepting such anomalies without revisiting the model, we’re not doing science — we’re doing belief maintenance.

I’m not dismissing that clouds form over cold ponds. I’m asking why this specific behavior (in a closed, reproducible kitchen experiment) is still unaddressed in thermodynamic theory.

That’s the real question.

 

[DaveE]

I’m asking why this specific behavior (in a closed, reproducible kitchen experiment) is still unaddressed in thermodynamic theory.

The specifics of your experiment are unaddressed, like what exactly is the temperature, pressure, and humidity at the location(s) of condensation. The phase states of water molecules are essentially completely understood. It's going to take a much more sophisticated experiment for you to convince anyone you have uncovered an anomaly.

I think you are confusing the small scale conditions with your large scale expectations of what will happen when you turn the heat on or off. The mystery lies in your experiment, not how water behaves. Predominantly, I would guess, in the temperature distribution and dynamics in that environment.

The mystery isn't in how the world works. The mystery is in what you have actually done with this stove top experiment. Don't just jump to revolutionary physics if experimental errors, lack of pertinent data, or fallacious assumptions are still a possibility. Experimental physics is hard to actually do.

 

[Dale]

But thermodynamic textbooks define visible vapor as a phenomenon occurring when water is close to boiling

Do they?

I usually see condensation discussed in terms of the dew point or relative humidity. Not in relation to boiling.

Can you provide an actual quote from the textbook that you are referencing?

What makes this observation challenging is not just that it happens — but that it contradicts the accepted explanation

I don’t think that it does. It sounds like you may be confusing evaporation with condensation, and maybe also boiling with evaporation. Clouds are condensation, not evaporation.

 

[jrmichler]

TL;DR Summary: Boiled 400ml water, turned off heat. Visible steam kept rising for 30+ minutes. At ~60°C, it was still ongoing. Classical thermodynamics doesn’t explain this. What’s the expected vapor time without heat?

I’m asking:
What would be the expected behavior in theory?

Here is some theory:

Liquid water has a vapor pressure that is a function of the water temperature. Some good search terms to find the relationship are water vapor pressure vs temperature and steam table water.

Air can hold an amount of water vapor that is a function of the air temperature. Air at a temperature will hold water vapor at partial pressures up to the vapor pressure of liquid water at that temperature. This is shown as the saturation line on a psychrometric chart (search the term).

When the water temperature is greater than the air temperature, the water vapor pressure is greater than the maximum partial pressure of water vapor in air. The excess water condenses to form fog. "Visible steam rising" is not steam, but fog. Steam is invisible, fog is visible.

 

[Dale]

This is the chart mentioned by @jrmichler It should be in any good thermodynamics textbook, as you mentioned. This table shows exactly what happens in your experiment:

Near the surface of the water, whether boiling or not, a certain mass of water is in the air, the humidity ratio. That is indicated as a horizontal line on this chart. This stays roughly constant until the humid air mixes with the ambient air.

As the water vapor rises from the pot, it cools, moving to the left on the appropriate blue line. When it lowers to the red 100% curved line, then the vapor condenses and becomes a visible cloud. It is not the boiling that produces the visible cloud, it is the cooling of the humid air.

As the air cools further it will lose moisture, going down and to the left following the red 100% line. If you reapply heat, the air will warm up, roughly going to the right on whichever blue line it is on at the time. As it leaves the 100% line condensation will cease, and the clouds will disappear. For as long as the humid air mixes with the ambient air before it cools down to the 100% red line.

• When I reapply heat, the vapor disappears (even as temperature increases).
• When I turn it off again, the vapor returns (while temperature is dropping).

According to the literature, this behavior should not occur.

No. This is precisely the behavior described in the chart.

All of the qualitative aspects of your experiment are reproduced on this chart. Furthermore, if you did all of your measurements precisely, then you would find that the theory reproduces the quantitative results also.

why this specific behavior (in a closed, reproducible kitchen experiment) is still unaddressed in thermodynamic theory

It isn’t. Either you have misunderstood a good source, or your source is not a good reference on thermodynamic theory.

 

[Pisica]

Air dissolves in water. That's why fish breathe in water.

The higher the temperature, the worse the air dissolves in the water.

When the water is slightly heated, the first bubbles that come to the surface of the water are the air dissolved in the water when the water was cold.

Do not confuse this with the bubbles that appear from boiling water.