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

In summary, when heating a pot of water, the temperature may not decrease slightly when it starts boiling because of the phase change energy required for water to reach its boiling point. This energy is called latent heat of phase change and is the same principle as when sweat evaporates to cool the body. The boiling process only occurs when the surrounding temperature is higher than the boiling point of the liquid, causing heat to transfer from the surroundings into the liquid. Other factors, such as atmospheric pressure, can affect the boiling point of water.
  • #1
TroyAthet
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When you heat a pot of water why wouldn't 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 isn't boiling the same principle and if so shouldn't the T of the remaining liquid get lower as the steam escapes into surroundings? Thanks for any help
 
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  • #2
TroyAthet said:
When you heat a pot of water why wouldn't 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 isn't boiling the same principle and if so shouldn't 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)
 
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  • #3
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.
 
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  • #4
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.
 
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  • #5
Isn't liquid hydrogen boiling at room temp or this another process?
 
  • #6
Ygggdrasil said:
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.
 
  • #7
Do you mean that temperature should slightly decrease during boiling, or when water starts to boil?
 
  • #8
snorkack said:
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
 
  • #9
Chris Riccard said:
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.
 
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  • #10
phinds said:
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?
 
  • #11
snorkack said:
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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  • #12
snorkack said:
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.

,
stock-photo-boiling-water-on-gas-flame-12093343.jpg
 

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  • #13
snorkack said:
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.
 
  • #14
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?
 
  • #15
snorkack said:
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?
 
  • #16
jim hardy said:
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.
 
  • #17
TroyAthet said:
When you heat a pot of water why wouldn't 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 isn't boiling the same principle and if so shouldn't 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:

2018.06.01.brew.pot.png


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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  • #18
snorkack said:
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
 
  • #19
jim hardy said:
many individual temperatures of discrete horizontal layers as in OM's experiment?
..., and the inverted (unstable/convecting) density profile as a consequence?
 
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  • #20
Bystander said:
..., and the inverted (unstable/convecting) density profile as a consequence?
exactly. Further, void fraction really upsets the density profile.
 
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  • #21
Bystander said:
..., and the inverted (unstable/convecting) density profile as a consequence?
I have no idea what that means.
jim hardy said:
exactly. Further, void fraction really upsets the density profile.
Nor this.

Does this new doodle convey what you two are saying?

2018.06.01.brew.pot.2.0.png


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]

jim hardy said:
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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  • #22
OmCheeto said:
new doodle convey what you two are saying?
Mmmm --- pretty much.
 
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  • #23
OmCheeto said:
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.

http://www.aquadyntech.com/watermolecule.html
upload_2018-6-2_21-36-30.png


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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  • #24
TroyAthet said:
When you heat a pot of water why wouldn't 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.
 

1. Why does the temperature of water remain constant during boiling?

The temperature of water does not decrease during boiling because boiling is a phase change, not a temperature change. When water reaches its boiling point, it begins to change from a liquid to a gas. During this process, the energy added to the water is used to break the bonds between water molecules, rather than increasing the temperature.

2. How does the energy added to water during boiling affect its temperature?

The energy added to water during boiling is used to break the bonds between water molecules, rather than increasing the temperature. This is known as the latent heat of vaporization. Once all the bonds are broken and the water has completely changed to steam, the temperature will begin to increase again.

3. Why is the temperature of boiling water the same at sea level and at high altitudes?

The temperature of boiling water remains the same at sea level and at high altitudes because the boiling point of water is determined by atmospheric pressure. At higher altitudes, there is less atmospheric pressure, which means water molecules can escape more easily, causing the water to boil at a lower temperature. However, since the energy added during boiling is used to break bonds, the temperature remains the same regardless of atmospheric pressure.

4. Does the temperature of water always remain constant during boiling?

No, the temperature of water does not always remain constant during boiling. If the water is contaminated with impurities or if it is in a container with a lid, the temperature may increase slightly above the boiling point. This is because the impurities or lid can act as nucleation sites, making it easier for bubbles to form and increasing the rate of boiling.

5. Can the temperature of water decrease during boiling if the heat source is removed?

Yes, the temperature of water can decrease during boiling if the heat source is removed. Once the heat source is removed, the water will no longer receive energy and the temperature will begin to decrease as the steam starts to condense back into liquid water. This is known as the latent heat of condensation.

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