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The Physics of Boiling Water

 
 
contrex
 
  1  
Reply Sun 5 Sep, 2010 11:45 am
@DrewDad,
DrewDad wrote:

contrex wrote:
Isn't steam normally called a vapour, not a gas?

Depends on whether or not you're a pedantic Brit, I suppose.

In the U.S., the gaseous phase of a substance is called a gas.


A vapour is a gas. Of course. But it was an American book that I looked in that said it is usual to use "vapor", if we wish to emphasize that we are talking about the gaseous phase, even though for some reason the condensed phase is considered "normal". I Briticised the spelling, that's all.


0 Replies
 
BumbleBeeBoogie
 
  1  
Reply Sun 5 Sep, 2010 12:56 pm
During 20 years of marriage, my husband insisted that the longer the water is boiled, the hotter it gets. I insisted that longer boiling just reduces the amount of water in the pan---lid or no lid.

I couldn't stand his opinion any longer, so I divorced him. Wink Razz Rolling Eyes

BBB
0 Replies
 
patiodog
 
  1  
Reply Sun 5 Sep, 2010 01:58 pm
At risk of drifting off into hyper pedantic land -- I think that most of what we see and feel coming off of boiling water is not technically steam, but rather hot water droplets, cooled to below the boiling point upon leaving the boiling body of water, suspended in the air -- a mist, or a vapor. If the bulk of the matter was truly gaseous, after all, we wouldn't be able to see it.
dyslexia
 
  1  
Reply Sun 5 Sep, 2010 02:03 pm
@patiodog,
law of thermodynamics says that any substance changing from one state to another loses heat ergo melting ice is cooler than solid ice and stream/water vapor is cooler than boiling water.
Fido
 
  1  
Reply Sun 5 Sep, 2010 02:05 pm
@littlek,
littlek wrote:

So, apparently my parents have had conflicting opinions about boiling water.

The goal: most efficient boilage.
The question is: which is more efficient - boiling with the lid on or off.

It seems obvious to me that the lid should be on to keep the heat in. But, my mother is of the opinion (instilled by her father) that if the lid is on, the internal pressure builds up such that the water takes longer to boil.

Does anyone want to weigh in on this issue?
Sure... The pressure difference is minimal... Water boil on a mountain top bols at a low temp, and to do the job, you need high temps which will build faster with the lid on...
0 Replies
 
georgeob1
 
  1  
Reply Sun 5 Sep, 2010 02:19 pm
@patiodog,
Not so. The term saturated steam is used for just such a mixture in which water vapor and water coexist at the same temperature. Superheated steam is the term used for pure water vapor heated above the boiling point (and to a higher pressure).

Water vapor forms from heated water at constant temperature (212 deg F at sea level) with the addition of heat or thermal energy. It condenses to pure water at the same constant temperature if heat or thermal energy is removed. Same goes for Freezing/melting of ice & liquid water - except at a lower, but also constant ,temperature.

The so-called "latent heat" released (per unit mass) when water vapor condenses is about 1000 times the specific heat or heat energy to raise the temperature of the same mass of water 1 deg. F. For liquid water freezing to ice the latent heat ratio is smaller, but still very large, about 150. That's how thunderstorms get their energy.
0 Replies
 
Thomas
 
  1  
Reply Sun 5 Sep, 2010 02:28 pm
@littlek,
littlek wrote:
Does anyone want to weigh in on this issue?

Nice pun! It does indeed depend on the lid's weight. If the lid is very heavy—and very well sealed so air cannot escape—your pot effectively turns into a pressure cooker, in which water takes a higher temperature to boil. But for realistic pots, with realistic lids, realistic sealage, and realistic heat conductivity through the metal, I'd side with you.

That said, one should never take an academic's word on anything that might happen in the Real World. One should run the experiment oneself instead.
0 Replies
 
littlek
 
  1  
Reply Sun 5 Sep, 2010 02:39 pm
The problem with running an experiment is that in the average kitchen you are neither likely to find 2 pots which are exactly the same nor be able to heat water under the same conditions.
georgeob1
 
  1  
Reply Sun 5 Sep, 2010 03:50 pm
@littlek,
True, but your original question was answered pretty well by other posters above.

If, as I suspect, you intend to add some food (say rice) to the water as soon as it boils, then you will remove the lid (if one is used) just as you detect the water is about to boil. This act will suddenly remove the pressure barrier from the escaping steam, lower the pressure at the water's surface and very likely start vigorous bulk boiling the moment you remove the lid. I believe this aspect of the matter turns the choice decisively to using a lid.

In fact engineers describe three stages of the approach to boiling;
1. So called "nucleate boiling" occurs when small vapor bubbles appear at the heated surface - and grow denser as heat addition continues.
2. "Departure from nucleate boiling" (an awkward engineering term) occurs when the small steam bubbles on the heated surface (at the bottom of the pot) grow larger and fairly suddenly start to rise to the surface.
3. Bulk boiling occurs when vapor bubbles form in the middle of the pot.

Basically this is an elaborate description of the rise in the water temperature from 212 deg F at the heated surface (but cooler above) to a uniform 212 deg F throughout. However, one can easily observe the three phases in a heated pot.

Important stuff if you're designing boilers or nuclear power plants. Not so much when you are preparing rice.
0 Replies
 
DrewDad
 
  2  
Reply Sun 5 Sep, 2010 04:35 pm
@dyslexia,
dyslexia wrote:

law of thermodynamics says that any substance changing from one state to another loses heat ergo melting ice is cooler than solid ice and stream/water vapor is cooler than boiling water.

Er... no.
0 Replies
 
patiodog
 
  3  
Reply Sun 5 Sep, 2010 04:42 pm
Thanks for the clarification, george.

Incidentally, I'm pretty sure the French, in their longstanding culinary tradition, have precise terms for all these states of about-to-boil/boiling-sedately/boiling-raucously/danger-to-anyone-in-immediate-vicinity. Where are our francophilic gastronomes?
0 Replies
 
rosborne979
 
  1  
Reply Sun 5 Sep, 2010 04:52 pm
@littlek,
littlek wrote:
The problem with running an experiment is that in the average kitchen you are neither likely to find 2 pots which are exactly the same nor be able to heat water under the same conditions.

That's probably true, but I bet it'll be hard to convince your parents with physics explanations too. Smile

0 Replies
 
Thomas
 
  1  
Reply Sun 5 Sep, 2010 06:24 pm
@littlek,
littlek wrote:

The problem with running an experiment is that in the average kitchen you are neither likely to find 2 pots which are exactly the same nor be able to heat water under the same conditions.

True. You'd have to run the experiment with the same pot on the same stove, in sequence. Once with the lid off, once with the lid on. No, actually make that twenty times each, because you want to filter out any once-off statistical flukes. Welcome to the real world of experimental physics. Now you know why doctoral theses in physics take 4-5 years, and why physicists fresh out of graduate school are always grumpy, frustrated, and cynical.

***

One more thought inspired by Georgeob1: for purposes of cooking, "nucleate boiling" is enough to tell you that the water is hot enough to have stff thrown into it. At this point, the boiling has barely any impact on volume, and thus on pressure. That's another reason the pressure-cooker effect should be minimal.
0 Replies
 
littlek
 
  2  
Reply Sun 5 Sep, 2010 07:14 pm
Ros - I'm not getting involved in THAT 45 year old argument. I was just curious and I thought this would make for a good thread topic (and it did!).
55hikky
 
  1  
Reply Sun 5 Sep, 2010 11:33 pm
@tsarstepan,
Quote:

I believe one needs the internal pressure to rise in order to help it boil faster. That's why pressure cookers can be dangerous and explosive if used wrong.

when there is high pressure, water actually doesn't boil faster (if the latter posts haven't already clarified for you)
there is a basic gas law pv=nrt
p=pressure, v=volume, n=# of molecules, r=constant #, T = temperature.
if temperature goes up, as it needs to for water to boil, one of two things must happen; pressure goes up, or volume goes up. so yes, high pressure actually does mean water boils faster.

have you ever heard of "making a cup of tea on top of a mountain" example?
on a high mountain, say everest; making tea is impossible; low atmospheric pressure allows water to boil at a low temperature so there's not enough heat to effectively brew tea from tealeaves.

the pressure cooker is dangerous since the volume of the water VAPOR builds as the temperature rises, and the pressure increases.
the whole purpose of this "pressure chamber pot" is not to "boil it faster", but so that the pressure of air above will force the water to have smaller volume; forcing flavor into the vegetables where there are less spice and flavoring, etc.

-55hikky
0 Replies
 
Arjuna
 
  1  
Reply Mon 6 Sep, 2010 05:37 am
@littlek,
littlek wrote:

Ros - I'm not getting involved in THAT 45 year old argument. I was just curious and I thought this would make for a good thread topic (and it did!).
Yes, plus it made me think through something I had assumed. I haven't observed it with water, but with soup that's boiling in a lidded pot, taking the lid off can suddenly make the soup boil out onto the stove... So I think it must be true that the lid makes the soup hotter and may cause a tiny increase in pressure.

Anybody happen to know if it's true that warm water freezes faster than cold water? Or is that another physics myth?
Thomas
 
  1  
Reply Mon 6 Sep, 2010 02:08 pm
@Arjuna,
Arjuna wrote:

Anybody happen to know if it's true that warm water freezes faster than cold water? Or is that another physics myth?

Physics myth. I saw the Mythbusters debunk it on TV. There's probably a link, but I cannot provide it because I'm traveling right now.
0 Replies
 
DrewDad
 
  1  
Reply Mon 6 Sep, 2010 03:18 pm
@tsarstepan,
tsarstepan wrote:

I believe one needs the internal pressure to rise in order to help it boil faster. That's why pressure cookers can be dangerous and explosive if used wrong.

No. The water boils at a higher temp, which causes food to cook faster, but it takes longer to come to a boil.
0 Replies
 
crumlev
 
  1  
Reply Sat 11 Jan, 2014 12:43 pm
@DrewDad,
aren't we concerned with the partial pressure of the water vapor over the liquid? is, bp is reached when the pressure of the water is equal to the pressure of the water vapor above it? In that case the lid would affect the bp to the degree that it traps the water vapor inside.
contrex
 
  1  
Reply Sat 11 Jan, 2014 01:28 pm
@crumlev,
This thread is over 3 years old.
0 Replies
 
 

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