[CliffordK (OP)]

I was just thinking today...
I've always been told that a covered pot boils faster than an open pot.  But, I've never tested it.

It would make a pretty simple science experiment.  One might have to test it a couple of times.

Now, the question...   WHY?

[Ethos_]

I was just thinking today...
I've always been told that a covered pot boils faster than an open pot.  But, I've never tested it.

It would make a pretty simple science experiment.  One might have to test it a couple of times.

Now, the question...   WHY?

Because a covered pot will retain heat more efficiently. As long as the lid doesn't confine the steam and allow pressure to build, the water will boil quicker.

[CliffordK (OP)]

What I believe the function of the lid is that it increases the partial pressure of water vapor above the water, which decreases the amount of water evaporating off of the top of the pot.

As water evaporates, it cools the water.  By increasing the partial pressure above the water, it would decrease the cooling effect from evaporation.

However, it may be difficult to discern the difference between simply retaining heat vs retaining water vapor.

[Ethos_]

What I believe the function of the lid is that it increases the partial pressure of water vapor above the water, which decreases the amount of water evaporating off of the top of the pot.

As water evaporates, it cools the water.  By increasing the partial pressure above the water, it would decrease the cooling effect from evaporation.

However, it may be difficult to discern the difference between simply retaining heat vs retaining water vapor.

Excellent points Cliff,.......I think you're right,  both those physical properties are likely responsible. But that also depends on the partial pressure remaining low enough. If it were to raise any substantial amount, it would raise the boiling point and increase the time and temperature necessary for the liquid to boil.

[CliffordK (OP)]

Not quite.
The boiling point is based on the pressure of the system, and not the concentration of water vapor above the liquid.  A rolling boil, of course, is the point where the bubbles occur near the heat source which is usually at the bottom of the pot.

I suppose one could think of the kettle as having 3 types of heat loss:

• radiative heat loss
• convective heat loss
• evaporative heat loss

Is there a way to calculate the contribution of each type of heat loss?  Perhaps the radiative heat loss would be similar with or without the lid as the lid could act like a larger heat sink.

Perhaps if one could precisely measure the amount of liquid lost between putting the pot on the burner, and it boiling, one could calculate the amount of heat lost due to evaporation, and presumably a difference between the covered and uncovered pot. 

[Ethos_]

Not quite.
The boiling point is based on the pressure of the system, and not the concentration of water vapor above the liquid. 

That point was made in my last post. Where I was agreeing with you was about the rate of evaporation also causing a cooling effect. When you used the term, "partial pressure" I was assuming that you only meant to contain the vapor above the liquid. That is the reason I also said; "If it were to raise any substantial amount, it would raise the boiling point." To be more precise, I should have said; Any increase in pressure would raise the boiling point.

Atmospheric pressure and heat are the major determining factor relative to the boiling point. If one creates a vacuum above the liquid water, it is possible to boil water at room temperature.

[alancalverd]

A loose cover will have no effect on absolute pressure but will clearly increase the partial pressure of water vapor above the liquid by preventing mixing with ambient air. But most importantly it reduces convective and radiative heat loss from the surface.

And yes, it does.

[Ethos_]

A loose cover will have no effect on absolute pressure but will clearly increase the partial pressure of water vapor above the liquid by preventing mixing with ambient air. But most importantly it reduces convective and radiative heat loss from the surface.

And yes, it does.

Well said alan........................

[bizerl]

A covered pot will always boil faster than an open pot UNLESS you are watching it, in which case it will never boil.  []

sorry, someone had to say it

[Ethos_]

A covered pot will always boil faster than an open pot UNLESS you are watching it, in which case it will never boil.  []

sorry, someone had to say it

Ha, ha, ha, good one my friend. I'll have to remember that one!

[Grimbo1]

old explores used to measure altitude by the boiling point of water, So the math must have been worked out a wile back.
Any one know the math ?

[alancalverd]

http://www.engineeringtoolbox.com/boiling-points-water-altitude-d_1344.html

[yor_on]

Thanks for that one Alan.
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