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# Boiling water: faster when you cover the pot?

## Boiling water: faster when you cover the pot? - Physics Forum

### Boiling water: faster when you cover the pot? - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

#1
11-09-2004, 01:11 PM
 Pascale Guest Posts: n/a
Boiling water: faster when you cover the pot?

Hi,

When I was in high school, my physics teacher taught us that contrary to
popular belief, water does NOT boil faster when you cover the pot. I
remember the explanation he gave had something to do with the pressure
changes that were not as efficient when the pot was covered. Or something
like that (it's been more than 20 years ago)...

Was he right? (Most people pretend that the opposite is true). Could anyone
come up with a "scientific" explanation?

Many thanks!

Pascale

#2
11-09-2004, 01:38 PM
 N:dlzc D:aol T:com \(dlzc\) Guest Posts: n/a
Boiling water: faster when you cover the pot?

Dear Pascale:

"Pascale" <[Only registered users see links. ]> wrote in message
news:xm3kd.134516\$df2.66028@edtnps89...

I believe you will find that:
1) if the cover is not hermetic, the water will boil faster, because the
lid (assuming opaque) provides a hot background that the water/pan bottom
will radiate less heat to.
2) if the cover impedes the steam release, such that pressure is built up,
the water will boil slower, since the boiling point will increase.

My two sense!

David A. Smith

#3
11-09-2004, 02:27 PM
 AJW Guest Posts: n/a
Boiling water: faster when you cover the pot?

>
In support of David's observation, and extending it: heat transfer via steam
loss and convection is greatly reduced when there's a lid on the pot.

If the pot lid weighs a pound and it covers a 8 inch diameter pot, that's about
50 sqare inches, it could raise the internal pressure inside the pot about 0.02
pounds a sqaure inch before the lid lifts, not enough to markedly increase the
water's boiling pressure.
#4
11-10-2004, 12:26 AM
 Peter Kupfer Guest Posts: n/a
Boiling water: faster when you cover the pot?

Pascale wrote:

Try it and report back.

No better proof than experiment!

Peter
#5
11-10-2004, 03:11 PM
 luriko Guest Posts: n/a
Boiling water: faster when you cover the pot?

On Wed, 10 Nov 2004 00:26:30 GMT, Peter Kupfer <[Only registered users see links. ]>
wrote:

He is right. It will not boil any faster. Remember the boiling point
of water is not a constant but a variable and a function of the
atmospheric pressure.
If you put a pot of water on to boil at sea level (lets use imperial
measurement) it will take x amount of BTUs to bring it to 212 deg. F
depending on the temperature and amount of water in the pot. At 212
deg.F the water will still not boil. It will then require latent heat.
This will taken additional 970 BTUs to boil the water. This will not
change regardless if the pot is covered or not. Except,,,if the cover
is sealed. In which case as the pressure in the pot rises then so will
the boiling point of the water. So, lets say the pressure in the pot
is at 15 psi then the water is going to be hotter than 212 deg.F . If
you have a strong enough pot and can let the pressure rise you will
eventually reach a point in water temperature that the water and steam
will exist at the same density. This is the critical point.
So, as you can see having the pot covered by a top makes not
difference..unless.. That cover can be secured pressure tight.
Just as a quick note the same thing also occurs in reverse. If you
reduce the pressure on the pot the boiling point then lowers. So, some
day if you climb to the summit of Mount Everest and decide to have a
cup of tea. Put your trusty pot of water and tea bag on a electric
burner and bring it to a rolling boil. Pour the boiling tea into your
cup. You will find it is only warm not piping hot.
#6
11-10-2004, 07:47 PM
 AJW Guest Posts: n/a
Boiling water: faster when you cover the pot?

>

Your analysis is mostly, but not entirely, correct. If you consider the thermal
'budget' you've accounted for the energy that goes into the water quite
accurately. You've not considered other routes of thermal energy uses. What
both I and David suggested is that thermal losses from the top surface of the
water are important (hold your hand above a pot of hot water and you'll agree)
and we claim a lid will interfere with convective and evaporative losses.

It gets better. It wasw suggested a watched pot never boils. I, on the other
hand, considering radiative heat losses, think a face watching the pot at 37
degrees C in hemisphere of otherwise 27 degree surfaces would reduce radiative
losses -- not by much, I agree -- so a watched pot should boil a little sooner
than an unwatched one.

So there, David!

AJW

#7
11-10-2004, 11:48 PM
 Peter Kupfer Guest Posts: n/a
Boiling water: faster when you cover the pot?

luriko wrote:

I did the reverse process in a vacuum pump last year just so I could see
it happen, and I left a thermometer in for the kids benefit. I am
assuming (and just double checking here) that this process provides no
safety benefit (like killing bacteria) because the temperature doesn't
get high enough. So, this would not help if you have a boil order in
effect, correct?

Peter
#8
11-11-2004, 01:22 AM
 N:dlzc D:aol T:com \(dlzc\) Guest Posts: n/a
Boiling water: faster when you cover the pot?

Dear Peter Kupfer:

"Peter Kupfer" <[Only registered users see links. ]> wrote in message
news:JNxkd.21132\$[Only registered users see links. ].prodigy. com...
....

The temperature should have dropped, as energy was "robbed" to vaporize
some water.

Freezing will also kill some bacteria.

At one factory, pressures in excess of 60,000 psi are used to sterilize
orange juice, and in excess of 100,000 psi to sterilize guacamole.
Diffusion though the cell walls is uncontrollable at those pressures...

I don't believe that pulling a vacuum will provide 100% kill, no. But the
condensate will be pretty free of bugs.

Some water supply systems on seacraft use an IC engine to pull a vacuum on
their own "cooling water jacket". The excess engine heat helps to boil
water at lower pressure, and the condensate is pretty free of salt.

David A. Smith

#9
11-11-2004, 01:23 AM
 luriko Guest Posts: n/a
Boiling water: faster when you cover the pot?

10 Nov 2004 19:47:21 GMT, [Only registered users see links. ]emove (AJW) wrote:

Here is the problem I have with the lid interfering with convective
and evaporative losses. When the water reaches the boiling temp, then
picks up the latent heat of evaporation the surface will have enough
energy to break away (steam). The steam will contact the sides and
lid. the latent heat will then convect to the lower temperature air
outside the pot. Remember "delta" Once the latent heat has been given
up the steam in the pot will condence back to water at 100C. So we
have only played around with thermal energy not evaporation or
convection.

Look at it this way.
Without a lid:
Abs press. psi - 14.696
temp. deg F - 212
Specific volume liquid - 0.01672
Specific volume - 26.80
Enthalpy liquid - 180.07
Enthalpy evap - 970.3
Enthalpy vapor - 1150.4
Entropy liquid - 0.3120
Entropy evap - 1.4446
Entropy vapor - 1.7566
Internal energy evap - 897.5

With a lid:
Abs press. psi - 14.696
temp. deg F - 212
Specific volume liquid - 0.01672
Specific volume - 26.80
Enthalpy liquid - 180.07
Enthalpy evap - 970.3
Enthalpy vapor - 1150.4
Entropy liquid - 0.3120
Entropy evap - 1.4446
Entropy vapor - 1.7566
Internal energy evap - 897.5

Same thing.
As for watching pots boil, is that like reading the telephone book?
;-)

#10
11-11-2004, 01:50 AM
 luriko Guest Posts: n/a
Boiling water: faster when you cover the pot?

On Wed, 10 Nov 2004 23:48:57 GMT, Peter Kupfer <[Only registered users see links. ]>
wrote:

Your right this process is not much use in sterilization or killing
bacteria. Simply because the thermal energy is not there. If you
reduce the absolute pressure down to 1.0 psi then your water will boil
at 101.74 deg F. So as you can see this would not be much good in
killing bacteria.
However if you were to raise the absolute pressure to ,lets say, 300
psi the water will boil at 417.33 deg F. Much better for
sterilization.
Nevertheless, this so called reverse process is of great importance in
the refrigeration cycle

 Tags boiling , cover , faster , pot , water

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