Can boiling and evaporation occur simultaneously?

In summary, evaporation and boiling occur due to the escape of high energy molecules. Evaporation can occur at or above the boiling point, and boiling takes place when the vapour pressure is greater than the atmospheric pressure plus any hydrostatic pressure at the bottom of the container. The latent heat of vaporisation limits the rate of boiling.
  • #1
coconut62
161
1
I understand the differences between evaporation and boiling as stated in secondary school textbooks.

But I would like to know, can evaporation occur at or above the boiling point?

I think that, since evaporation is the escaping of high energy molecules, then when the water is boiling, there may also be some molecules whose kinetic energy is higher than others, thus they escape faster. It's not like all molecules vaporize at the same time, after all.

Or evaporation is just a word specially used "for vaporisation below the boiling point"?

Thank you.
 
Physics news on Phys.org
  • #3
What was stated in your textbooks that you found easy to understand?
Do you now have doubts about what you read?
 
  • #4
coconut62 said:
I understand the differences between evaporation and boiling as stated in secondary school textbooks.

But I would like to know, can evaporation occur at or above the boiling point?

I think that, since evaporation is the escaping of high energy molecules, then when the water is boiling, there may also be some molecules whose kinetic energy is higher than others, thus they escape faster. It's not like all molecules vaporize at the same time, after all.

Or evaporation is just a word specially used "for vaporisation below the boiling point"?

Thank you.

I can sympathise with your confusion and it could be just down to the 'classifying' problem. What we actually observe is due to a number of factors. Water is always evaporating (losing molecules) at any temperature. There is a positive 'vapour pressure'. If the surrounding air is not 'saturated' then there will be a net loss of water because more molecules are lost than returning. If new, 'dry' air is passing over the surface, then the evaporation rate is higher because fewer molecules get back onto the surface.
Boiling takes place when the vapour pressure is greater than the atmospheric pressure plus any hydrostatic pressure at the bottom of the container (or the position of the heating element). Bubbles can form around an element when enough vapour is produced fast enough. That's boiling - but I suppose you could just call it very rapid evaporation - with no balancing condensation.
There's another issue, here, which comes into play. Evaporation / boiling takes energy and all the water won't boil at once - unless there is a fantastic rate of power supplied. This 'latent heat of vaporisation' is what limits the rate of boiling. And, of course, the boiling takes place where the heat is being supplied - usually at the bottom. You can get 'explosive' boiling if the energy is supplied quick enough.
 
  • #5
sophiecentaur said:
Boiling takes place when the vapour pressure is greater than the atmospheric pressure plus any hydrostatic pressure at the bottom of the container (or the position of the heating element).
Plus the capillary pressure on the bubbles forming.
sophiecentaur said:
Bubbles can form around an element when enough vapour is produced fast enough. That's boiling - but I suppose you could just call it very rapid evaporation - with no balancing condensation.
There's another issue, here, which comes into play. Evaporation / boiling takes energy and all the water won't boil at once - unless there is a fantastic rate of power supplied.
It still wouldn´t be at once. As the temperature of water increases, bubbles will get easier to nucleate.
sophiecentaur said:
This 'latent heat of vaporisation' is what limits the rate of boiling.
The rate of power supply does not limit the rate of boiling. Boiling liquid can rapidly cool towards its boiling point, even though the heat supply was slow and remains slow.
sophiecentaur said:
And, of course, the boiling takes place where the heat is being supplied - usually at the bottom.
Where heat is being supplied is the hottest part of liquid, due to heat conduction being limited by temperature gradients. So it is the most likely place to boil. But it does not need to be the place where the liquid can boil. For one, the liquid might have higher boiling poing there, for example due to higher pressure. For another, the part where bubbles are nucleated might be elsewhere than where heat is supplied.
sophiecentaur said:
You can get 'explosive' boiling if the energy is supplied quick enough.

Or if the energy builds up above boiling point due to lack of bubble nucleatrion sites. Explosive boiling is a known hazard with test tubes, microwave ovens etc.

And evaporation can easily happen well above boiling point. If a vessel contains overheated liquid which is unable to form bubbles due to lack of nucleation sites in the liquid and bottom, it still can evaporate from free surface to air.
 
  • #6
'Explosive' boiling is a function of surface tension! The pressure in tiny bubbles is very large and bubbles will not form unless there are nucleation sites...dirt, scratches etc, these are places where bubbles in the form of trapped gas already exist.
Anti bump granules are sometimes added to liquids to act as nucleation sites.
In a similar way, but nothing to do with evaporation, a clean glass of lager produces very few bubbles...drop a sugar cube in and see the release of trapped (dissolved) gas once nucleation sites are available. Then drink the lager.
 
  • #7
snorkack said:
Plus the capillary pressure on the bubbles forming.

It still wouldn´t be at once. As the temperature of water increases, bubbles will get easier to nucleate.

The rate of power supply does not limit the rate of boiling. Boiling liquid can rapidly cool towards its boiling point, even though the heat supply was slow and remains slow.

Where heat is being supplied is the hottest part of liquid, due to heat conduction being limited by temperature gradients. So it is the most likely place to boil. But it does not need to be the place where the liquid can boil. For one, the liquid might have higher boiling poing there, for example due to higher pressure. For another, the part where bubbles are nucleated might be elsewhere than where heat is supplied.


Or if the energy builds up above boiling point due to lack of bubble nucleatrion sites. Explosive boiling is a known hazard with test tubes, microwave ovens etc.

And evaporation can easily happen well above boiling point. If a vessel contains overheated liquid which is unable to form bubbles due to lack of nucleation sites in the liquid and bottom, it still can evaporate from free surface to air.

Yup. That's all good stuff when dealing at the next level up. Trouble is, there's no end to these explanations and I wanted to keep it as simple as possible.
 
  • #8
I'm sorry if I'm hijacking this thread, but I think my question is relevant here: Why is atmospheric pressure an important consideration in the case of boiling? Water vapor specifically, is lighter than air, so by Archimedes principle, it should be able to escape liquid water regardless of the air pressure?

I'm sure I'm missing a very important detail here. Please excuse my ignorance.
 
  • #9
It can: that's what evaporation is. But boiling involves bubbles of water vapor inside a liquid. The pressure inside those bubbles is determined by atmospheric pressure and depth.
 
  • #10
russ_watters said:
It can: that's what evaporation is. But boiling involves bubbles of water vapor inside a liquid. The pressure inside those bubbles is determined by atmospheric pressure and depth.

So,

1. Evaporation does not involve bubbles inside the liquid; the molecules just simply escape from the surface.
2. Boiling must involve the formation of bubbles.

Are these both correct?
 
  • #11
coconut62 said:
So,

1. Evaporation does not involve bubbles inside the liquid; the molecules just simply escape from the surface.
2. Boiling must involve the formation of bubbles.

Are these both correct?
Yes. Those are the definitions.
 

1. What is the difference between boiling and evaporation?

Boiling is a process in which a liquid changes into a gas at a specific temperature, while evaporation is the process of conversion of a liquid into a gas at any temperature.

2. Can boiling and evaporation occur simultaneously?

Yes, boiling and evaporation can occur simultaneously. When a liquid is heated, the molecules at the surface gain enough energy to escape into the air as gas, while the remaining liquid continues to heat up and eventually reaches its boiling point.

3. Is boiling and evaporation the same thing?

No, boiling and evaporation are not the same thing. Boiling occurs at a specific temperature, while evaporation can occur at any temperature. Boiling also involves the formation of bubbles, while evaporation does not.

4. Can boiling and evaporation happen at room temperature?

Yes, evaporation can occur at room temperature. However, boiling can only occur at a specific temperature, which is different for each liquid. For example, water boils at 100 degrees Celsius, while alcohol boils at 78.37 degrees Celsius.

5. How does boiling and evaporation affect the properties of a liquid?

Boiling and evaporation both cause the liquid to lose some of its mass, as the molecules escape into the air as gas. This can lead to a decrease in the volume and an increase in the concentration of the remaining liquid. Boiling can also cause changes in the chemical properties of the liquid, such as the breakdown of compounds or the release of gases.

Similar threads

  • Other Physics Topics
Replies
13
Views
2K
Replies
32
Views
1K
Replies
9
Views
4K
Replies
3
Views
1K
  • Other Physics Topics
Replies
3
Views
1K
Replies
4
Views
8K
  • Introductory Physics Homework Help
Replies
4
Views
3K
Replies
8
Views
2K
  • Introductory Physics Homework Help
Replies
3
Views
5K
Back
Top