# Why doesn't the temperature of water slightly decrease during boiling?

#### TroyAthet

When you heat a pot of water why wouldnt the water temp decrease slightly when it starts boiling? Isnt boiling a type of evaporation? In evaporation the molecules near surface have enough KE to escape into the gas phase, which then lowers the T of the remaining liquid. That is how sweating cools the body right? So isnt boiling the same principle and if so shouldnt the T of the remaining liquid get lower as the steam escapes into surroundings? Thanks for any help

#### phinds

Gold Member
When you heat a pot of water why wouldnt the water temp decrease slightly when it starts boiling? Isnt boiling a type of evaporation? In evaporation the molecules near surface have enough KE to escape into the gas phase, which then lowers the T of the remaining liquid. That is how sweating cools the body right? So isnt boiling the same principle and if so shouldnt the T of the remaining liquid get lower as the steam escapes into surroundings? Thanks for any help
You have lost track of (or perhaps are not familiar with) the phase change energy that it takes to make water go from 100 degree C to BOILING at 100 degrees C. THAT is the energy that is given up as the water boils, not the energy that it took to get it from 99 degrees to 100 degrees. This energy is called latent heat of phase change and yes it IS the same as when your sweat evaporates (it's the same phase change; liquid to gas)

#### jim mcnamara

Mentor
You may also google and find additional explanations, although @phinds explanation is pretty good. - there is another term for it as well: latent heat of vaporization.

#### Ygggdrasil

Gold Member
Water (or most any other liquid) will boil only when being heated, so loss of energy from the vaporization process is countered by absorption of heat from the surroundings. If you take a boiling pot of water off of the stove, the boiling will stop fairly quickly for the reasons you and others in the thread have mentioned.

#### Chris Riccard

Isn't liquid hydrogen boiling at room temp or this another process?

#### Let'sthink

Water (or most any other liquid) will boil only when being heated, so loss of energy from the vaporization process is countered by absorption of heat from the surroundings. If you take a boiling pot of water off of the stove, the boiling will stop fairly quickly for the reasons you and others in the thread have mentioned.
I agree with this answer.

#### snorkack

Do you mean that temperature should slightly decrease during boiling, or when water starts to boil?

#### phinds

Gold Member
Do you mean that temperature should slightly decrease during boiling, or when water starts to boil?
No one should believe either one of those things. See post #2

#### Ygggdrasil

Gold Member
Isn't liquid hydrogen boiling at room temp or this another process?
Hydrogen's boiling point is about -250°C, and any liquid should start to boil when the surrounding temperature is greater than its boiling point. Because the temperature of the liquid hydrogen is below the temperature of its surroundings, heat is being transferred from the surroundings to the liquid hydrogen to support is vaporization.

Similarly, when you place water on a stove to get it to boil, the heating element of the stove is at a higher temperature than the water in the pot, causing heat to move from the heating element into the water to power the boiling process.

#### snorkack

No one should believe either one of those things. See post #2
Water boils at 100,00 degrees at 760 mm Hg. But at 101,00 degrees at 776 mm Hg
16 mm Hg has the same weight as 22 cm of cold water, or 23 cm of boiling water.
In a cauldron 46 cm deep, the boiling point of water is 100,00 degrees at the surface, but 102,00 degrees at the bottom.
If a cauldron is heated to boiling, but has not started to boil because the temperature at the bottom is 101,99 degrees but the boiling point at the bottom is 102,00 degrees, what is the temperature at the middle of the cauldron, 23 cm from bottom and surface?

#### phinds

Gold Member
Water boils at 100,00 degrees at 760 mm Hg. But at 101,00 degrees at 776 mm Hg
16 mm Hg has the same weight as 22 cm of cold water, or 23 cm of boiling water.
In a cauldron 46 cm deep, the boiling point of water is 100,00 degrees at the surface, but 102,00 degrees at the bottom.
If a cauldron is heated to boiling, but has not started to boil because the temperature at the bottom is 101,99 degrees but the boiling point at the bottom is 102,00 degrees, what is the temperature at the middle of the cauldron, 23 cm from bottom and surface?
I think you are bringing in complications that are irrelevant to the original question.

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#### jim hardy

Gold Member
2018 Award
If a cauldron is heated to boiling, but has not started to boil because the temperature at the bottom is 101,99 degrees but the boiling point at the bottom is 102,00 degrees, what is the temperature at the middle of the cauldron, 23 cm from bottom and surface?
Depends on how well it's mixing, doesn't it ?

There's nucleate boiling and bulk boiling that you can observe on a stovetop. Convection mixes it until bulk boiling takes over.

,

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#### Let'sthink

Do you mean that temperature should slightly decrease during boiling, or when water starts to boil?
It has tendency to fall but is maintained due to supply from heat source.

#### snorkack

If you have a cauldron which is not boiling because the boiling point at the bottom is 102,00 degrees but water temperature is 101,99 degrees, then the temperature at the surface is 100,00 degrees, because that is the boiling point there and water molecules freely leave the smooth surface into air, cooling the surface by evaporation. And a thin surface layer by conduction.
But in the middle of the cauldron, you could well have water whose temperature is 101,99 degrees, boiling point 101,00 degrees but which is not boiling.
It is not in contact with air (that's 23 cm away), it is not in contact with steam bubbles rising up from bottom (bottom is not boiling) and it is not nucleating any steam bubbles because of surface tension.
When the bottom heats to 102,00 degrees and does start to boil, the water in the middle of the cauldron does come to contact with steam bubbles rising from bottom, rapidly evaporates steam to join these bubbles and is by latent heat rapidly cooled to its boiling point of 101,00 degrees.
Would you expect water to overheat before it starts to boil, and undergo a slight cooling to its boiling point when it does start to boil?

#### jim hardy

Gold Member
2018 Award
Would you expect water to overheat before it starts to boil, and undergo a slight cooling to its boiling point when it does start to boil?
I wouldn't expect that. I'm no chemist, but i'd expect formation and collapse of microscopic bubbles at the heat transfer surface. Probably that's the hiss you hear in a coffeepot before bulk boiling commences?

#### snorkack

I wouldn't expect that. I'm no chemist, but i'd expect formation and collapse of microscopic bubbles at the heat transfer surface.
But the boiling point is higher at the bottom, because of higher pressure.

#### OmCheeto

Gold Member
When you heat a pot of water why wouldnt the water temp decrease slightly when it starts boiling? Isnt boiling a type of evaporation? In evaporation the molecules near surface have enough KE to escape into the gas phase, which then lowers the T of the remaining liquid. That is how sweating cools the body right? So isnt boiling the same principle and if so shouldnt the T of the remaining liquid get lower as the steam escapes into surroundings? Thanks for any help
Your question is as complicated as my pictorial answer:

Of course, 330 K is not the boiling point of water, but I don't really think it's that big a deal.

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#### jim hardy

Gold Member
2018 Award
But the boiling point is higher at the bottom, because of higher pressure.
So what ?
Have you never poured water out of a coffeepot that's still boiling?
As you tip it pressure everywhere beneath the surface is lowered because of reduced height, any bubbles expand and you get a rush of boiling coffee that spills out around the top of the pot..
Add just a couple spoonfuls of water before pouring and it subcools the pot enough that doesn't happen.

Paying attention to little everyday experiences and questioning 'why' is good for our understanding.

Which 'water temperature' are you talking about ? A single average temperature of the entire contents, or many individual temperatures of discrete horizontal layers as in OM's experiment?

old jim

#### Bystander

Homework Helper
Gold Member
many individual temperatures of discrete horizontal layers as in OM's experiment?
..., and the inverted (unstable/convecting) density profile as a consequence?

#### jim hardy

Gold Member
2018 Award
..., and the inverted (unstable/convecting) density profile as a consequence?
exactly. Further, void fraction really upsets the density profile.

#### OmCheeto

Gold Member
..., and the inverted (unstable/convecting) density profile as a consequence?
I have no idea what that means.
exactly. Further, void fraction really upsets the density profile.
Nor this.

Does this new doodle convey what you two are saying?

def 1, per wiki; "Boiling is the rapid vaporization of a liquid..." [ref 1]
def 2, per wiki; "When a liquid reaches its boiling point bubbles of gas form in it which rise into the surface and burst into the air. This process is called boiling." [ref 2]

or many individual temperatures of discrete horizontal layers as in OM's experiment?
I actually gave up the actual experiment when the surface temperature of my pot reached the liquification point of the glue on my masking tape holding the black craft paper to the pot, and everything kind of fell apart.

Prior to the "black craft paper" addition, I knew something was wrong, when I touched the pot, when my laser guided thermometer said the temperature should have been comfortable, and I burned the back of my fingers. Which is why I tried to experimentally determine the emissivity of my pot. (Did anyone check my maths on that?)

Engineeringtoolbox gave values that were a full magnitude different:
Stainless Steel, weathered 0.85
Stainless Steel, polished 0.075​

Guessing my pot is closer to "weathered" than "polished".

My favorite discovery; "The nucleate boiling process has a complex nature. A limited number of experimental studies provided valuable insights into the boiling phenomena, however these studies provided often contradictory data due to internal recalculation (state of chaos in the fluid not applying to classical thermodynamic methods of calculation, therefore giving wrong return values) and have not provided conclusive findings yet to develop models and correlations. Nucleate boiling phenomenon still requires more understanding." [ref 3]

As that makes me feel less stupid.

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#### jim hardy

Gold Member
2018 Award
Does this new doodle convey what you two are saying?
I think it does. You added flow, which is convection ie hot fluid rises, so there's mixing.

Maybe we think of 'boiling' in not the same sense..
I think of water as highly polar molecules. The O at one end hogs the electrons so you might think of individual molecules as little electric dipoles, minus at the O end and plus out at the H's.. They're attracted to one another (almost like static electricity attracts a plastic dry cleaner's bag to your arm) hence water's remarkable surface tension and latent heat of vaporization.

Boiling to me means pushing the molecules apart from one another, overcoming those attractive electrostatic forces labelled 'Hydrogen Bonds' between molecules.
That's energy that doesn't show up as temperature, it's a different form of internal energy. Remember temperature is related to kinetic energy of the molecules ie velocity. see http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/kintem.html

Of course when a group of water molecules have separated you now have a steam bubble that's a lot less dense so it'll float.
When it's violent enough to see motion we describe that as "Boiling" (or perhaps "Roiling" ).

But to my alleged brain, boiling commences at the molecular level well before we see it.

Ahh, terminology. You'll love taking Thermodynamics.

Does that help any ?

old jim

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#### Rive

When you heat a pot of water why wouldnt the water temp decrease slightly when it starts boiling?
Mostly, because the chaotic environment of the boiling liquid usually averages out the local temperature differences on larger scale. The result is, that the evaporated liquid will contain the excess heat and you will have a boiling liquid mass at ~ 100 degree temperature.

When it is not chaotic, boiling might noticeably decrease the temperature of the liquid. For example overheated water goes back to 100 degree after a very intensive boiling period. Or you can force boiling by reducing pressure, and this will also decrease the temperature of the liquid.

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