Thread: The question of boiling water...

Not sure if this should go in chemistry of physics, since it's a bit of physical chemistry.... I've been thinking about this since my dad and i were cooknig one night.

Liquid water can't exceed temperature of 100 degrees celsius under standard pressue conditions. At 100 degrees it becomes a gas. I'm cooking potatoes and want them to cook as fast as possible. My water boils at 8 on the stove scale(which goes up to 10). If i put the stove to 9 or 10, the water boils more rapidly. ie more of a rolling boil.

Can any of the liquid water in the pot exceed 100 degrees? Will a rolling boil cook potatoes faster, or are both pots the same temp(one is jsut evaporating faster)?

For some reason i have it in my head that the water can get hotter than 100 degrees, but if it's not under pressure it shouldn't be able to...right? Does volume play a role?

There seem to be two issues that might allow the temperature to rise slightly above nominal boiling point.
Water at the bottom of the pot is under more pressure than the water at the surface and so will have a slightly higher boiling point.
I think (though I am not 100% certain) that some stimulus is needed to inititate the vaporisation of the water. (Comparable with the seeding of crystal growth.) In very pure water and distant from the walls of the container such stimuli would be limited.
Both of these effects would be very small - and frankly the second one might be wholly imaginary.

Originally Posted by Ophiolite

Both of these effects would be very small - and frankly the second one might be wholly imaginary.

The first one is true but equally for the slowly boiling as the rapidly boiling water. I don't think you can speed it up by boiling faster, but a pressure cooker works well.

The exception would be when you just have nucleate boiling at the bottom of the pan and the bulk of the liquid hasn't started boiling yet.

The following is a quote from a heat transfer site that I use constantly:

“In incipient boiling sufficient superheat is present to activate some nucleation cavities and heat is transferred by a combination of convection and nucleate boiling. Nucleation sites first become activated at a certain [temperature difference between the metal surface and the local saturation temperature of the fluid]. This is a function primarily of the size distribution of cavities in the surface and is difficult to generalize.”

In experiments with boiling on different surfaces it has been found that a highly polished gold surface can sustain large amounts of superheat without boiling occurring, while a rusty steel surface will start the fluid boiling with very little superheat. A stainless steel saucepan would lie somewhere in between.

Yeah, but as soon as you add the potatoes to the pot you'll be providing plenty of rough surface area for gas bubbles to nucleate.

I think what they are talking about there is the metal gets above the boiling point without boiling the water at the metal/liquid interface. That doesn't get the bulk of the liquid superheated.

Superheating the liquid can happen in a microwave oven. You heat your coffee in the microwave and all of a sudden - pop - your coffee is all over the inside of the oven. Or if you take it out of the microwave it could explode in your face.

Let's say you have some cold potatoes in the pot. They can cool the liquid bulk temperature below the boiling point while you have some nucleate boiling happening at the bottom of the pot where it is being heated. But once you get a good rolling boil going on, it won't help to boil it any faster.

Yes, agreed.

I was answering the question "Can any of the liquid water in the pot exceed 100 degrees?" and the answer is yes.

I think adding salt increases the boiling temperature of water by a few degrees also.

I think salt will act as impurity and increase the bp. Am I right?

Adding solutes to a solution changes the Colligative properties of that solution.

in this case, salt and water, creates a solution with different Colligative properties than the original water because there's more molecules in the water as such.

Colligative properties include : Increasing boiling point, decreasing freezing point, a change in vapour pressure and change in osmotic pressure.

I think that's why salt increases boiling point, so yeah you could say its due to it being an impurity.

Edit : also, adding salt to the pot, while raising the temp, wouldn't have much affect on the speed of cooking, as the time taken to raise a small temp, would probably cancel the little bit of extra heat cooking that little bit faster. to get a raise equal to that of a pressure cooker, you'd have VERY salty potatoes.

Pure water in a microwave will reach around 400^0c.

why?

Originally Posted by Ieshwar

why?

http://uk.youtube.com/watch?v=9SOg5jzini0&NR=1

The water must be pure and distilled. Not one mineral must be in it. Water boils in a kettle because it attaches to other elements in it.

water boils in a kettle because the "element" ie the heating element raises it to boiling temperature (the temp at which vapor pressure of a liquid = the vapor pressure of the environment) it reaches this temperature because heat has been transferred to the water from the hot element.

as far as my knowledge goes any how, i dont think it has anything to do with attachment to other elements.

correct me if im wrong.

I think svwillmer is referring to superheating.
http://en.wikipedia.org/wiki/Superheating
It has nothing to do with the purity of the water, but rather the lack of nucleation sites, and I doubt very much if it can get to 400 C.