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The Physics of Boiling Water

 
 
littlek
 
Reply Sat 4 Sep, 2010 03:50 pm
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?
 
tsarstepan
 
  1  
Reply Sat 4 Sep, 2010 04:02 pm
@littlek,
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.
fresco
 
  1  
Reply Sat 4 Sep, 2010 04:10 pm
@littlek,
With the lid on, less heat escapes so it should take less time. Unless you have a pressure cooker the boiling point is unlikely to be raised significantly, but even if it is, the water will have actually reached 100 deg faster assuming this is the required "cooking temperature". i.e. "boiling point" is an irrelevance, "cooking temperature" is the significant issue.
0 Replies
 
dyslexia
 
  1  
Reply Sat 4 Sep, 2010 04:11 pm
no doubt about it --- Lid On
0 Replies
 
Mame
 
  1  
Reply Sat 4 Sep, 2010 04:18 pm
It's also very important that you not watch the water - that's the same or even worse as having the lid off.
fresco
 
  1  
Reply Sat 4 Sep, 2010 04:22 pm
@Mame,
And keeping the lid on stops you watching it ... it has nothing to do with physics ! Laughing
0 Replies
 
tsarstepan
 
  1  
Reply Sat 4 Sep, 2010 04:26 pm
@littlek,
Factors That Affect the Boiling Point

Pressure: when the external pressure is:
less than one atmosphere, the boiling point of the liquid is lower than its normal boiling point.
equal to one atmosphere, the boiling point of a liquid is called the normal boiling point.
greater than one atmosphere, the boiling point of the liquid is greater than its normal boiling point.
The following graph shows the boiling point for water as a function of the external pressure. The line on the graph shows the normal boiling point for water.
http://www.chem.purdue.edu/gchelp/liquids/bpgraph.gif

Closed lid = greater pressure inside pot then outside. Water boils faster.

No lid = pressure outside is = to pressure inside pot. Water boils at 100 degrees Celsius.

dyslexia
 
  2  
Reply Sat 4 Sep, 2010 04:45 pm
Lady Diane keeps a lid in the drawer under the micro-wave, she says it's for medicinal purposes (and brownies)
littlek
 
  1  
Reply Sat 4 Sep, 2010 04:46 pm
@dyslexia,
snort
0 Replies
 
roger
 
  1  
Reply Sat 4 Sep, 2010 06:32 pm
@tsarstepan,
Around here it boils at 202f. Keep your funny numbers. That's 5,000 feet, by the way
0 Replies
 
patiodog
 
  1  
Reply Sat 4 Sep, 2010 06:46 pm
@tsarstepan,
Misinterpretation of graph, actually. Increasing pressure increases the temperature at which water boils. All else being equal (that is, if we ignore the fact that heat escapes with steam/vapor as water is heated with the lid off), it will take longer to reach the boiling point given an equivalent heat source, because the boiling point is higher. The real point of the pressure cooker (or the autoclave, in medical and scientific circles) is that water gets hotter without boiling at higher pressures, thereby making tender meat faster and killing some of the nastier sporulating bacteria that like to make messes of surgeries and experiments.

Nonetheless, I concur with the lid on crowd, because you lose much less heat (and water) to convection. In fact, I'm about to go check on the potatoes that are boiling under a lid at this very moment. (Since the lid is glass, however, I am still able to watch if so inclined.)
Arjuna
 
  1  
Reply Sat 4 Sep, 2010 06:54 pm
@patiodog,
Yea, theoretically water should reach the boiling point faster with the lid off.

But it would hard to check that unless you have a glass pot so you can see the bubbles forming at the bottom. Otherwise, you'll remove the pressure when you take the lid off.
0 Replies
 
DrewDad
 
  1  
Reply Sat 4 Sep, 2010 08:17 pm
@littlek,
Unless the lid is that of a pressure cooker, the internal pressure will not increase a significant amount, and will not affect the point at which the water boils.

The thermal efficiency gained by having the lid on outweighs any effect a lid might have on the boiling point.

The pressure under the lid can only equal atmospheric pressure + (weight of lid)/(surface area of lid). A four-ounce lid on an 8 inch diameter pot can create a pressure differential of .005 lbs/square inch. Sea-level atmospheric pressure is something like 15 lbs/square inch.
Arjuna
 
  1  
Reply Sat 4 Sep, 2010 09:54 pm
@DrewDad,
Oh yea, very true. But with my pot lids the pressure would also have to overcome friction between the lid and the pot... there's about a half inch vertical lip on the lid that contacts the pot and creates a seal... not a pressure cooker seal, but I avoid closing the lid and turning the heat up because of what will happen when I lift the lid.
DrewDad
 
  1  
Reply Sat 4 Sep, 2010 09:59 pm
@Arjuna,
Arjuna wrote:
friction between the lid and the pot...

Minimal. Especially since it's being lubricated by the condensation of the steam.

As for the effect of opening the lid, the gas inside is almost pure steam.
contrex
 
  1  
Reply Sun 5 Sep, 2010 03:26 am
@DrewDad,
DrewDad wrote:

Arjuna wrote:
friction between the lid and the pot...

Minimal. Especially since it's being lubricated by the condensation of the steam.

As for the effect of opening the lid, the gas inside is almost pure steam.


Isn't steam normally called a vapour, not a gas? I know a vapour is a kind of gas, but it is normal to call the gaseous phase of a substance (e.g. water) "vapour" if we wish to emphasize that we are talking about the gaseous phase, even though for some reason the condensed phase is considered "normal". Anyhow, what is above the boiling water in a saucepan is tiny droplets of condensed water vapour.


maxdancona
 
  2  
Reply Sun 5 Sep, 2010 08:44 am
OK, this thread is making a common mistake in discussions about science. You are comparing a significant effect with a very slight effect and considering them equal.

1) Yes, it is absolutely true that increasing the pressure will raise the boiling point of water. You would see this happen if you welded the lid on the pot (or used a pressure cooker).

2) Putting a lid on a pot increases the pressure very very slightly which is why the lid doesn't come off. The raise in pressure is so slight that the raised boiling point is insignificant compared to the effect below.

3) Putting a lid on a pot keeps heat energy from escaping. This means that more energy stays in the water making it hotter quicker (and less is put in the air). This is a significant effect.

Without question water will boil much faster with a lid on.

fresco
 
  1  
Reply Sun 5 Sep, 2010 09:12 am
@maxdancona,
I seem to have read this somewhere above. Smile
0 Replies
 
rosborne979
 
  1  
Reply Sun 5 Sep, 2010 10:50 am
@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.

I doubt the difference in boiling time is very significant either way, but there's an easy way to tell for sure... try it with two equal amounts of water in similar containers and see what happens.
0 Replies
 
DrewDad
 
  3  
Reply Sun 5 Sep, 2010 11:39 am
@contrex,
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.
 

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