When cooking which part is the hottest, top or bottom of liquids ?

Admited that the flame is below the Utensil but heat rises up.
So is the liquid boiling hotter at the top or at the bottom at constant flame ?

So If I am cooking peas with potato (just an example) peas taking longer time to cook, should I place them above potato or below.

I want to put peas in the hotter region of the boling process to save overall cooking time.

Thanks in advance.

 

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Convection currents are caused in the liquid by hotter material rising, so the situation is complex, in some places the cooler material is sinking. On average, I would suspect it's hotter at the bottom. Do you have a cooking thermometer?

 

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no, but good suggestion

 

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there will be no noticable difference between the bottom and the top... unless the container you are using is very very tall.....

just put the peas in five minutes earlier than the potato.

-MT

 

ya sure, no wonder things burn at the bottom while cooking

ps> did u check the direct democracy thread.

 

So U mean water can never exceed 100 degrees ?

but on full flame the water boils very furociously, and on mild flame the water does boil but take longer time to reach boiling point, so are both at 100 degrees ?

 

if water boils its always 100c, no matter how furiciously, however, water can exceed 100c few degrees if there is no boiling point. GOOGLE LINK
But organic material such as putatos and beans have a lot of boiling points so...
Another option is to seal the container so the pressure keeps water liquid... but I think it might be a bit dangerous.

 

What about this : how high would you need a column of water to affect the boiling temperature at the bottom? Or does it affect it at all (I'm pretty sure it has to). After many convection cycles close to 100C, does the whole thing boil at once?

 

I have to disagree with you on this, James. I agree that the water is at 100 C regardless of the burner setting. Cooks may disagree on whether to add salt or oil, but thay all agree a rapid boil is needed. Why? (1) Adding pasta reduces the water temperature and makes it stop boiling for a bit. The water will come back a a boil faster if the heat supply is greater. (This argument applies more so to an electric range than a gas range.) (2) The outer layer of pasta is nasty and sticky. The roiling currents of a rapid boil wash this outer layer away. (3) Unstirred pasta binds to itself very quickly. A rapid boil helps keep the pieces of pasta in motion and from sticking to each other.

To answer your first question, the column of water raises the pressure, raising the boiling point. The pressure at the bottom of the column will rise above the critical pressure (217.7 atm) if you make the column tall enough.

To answer your second question, merely raising the entire volume to 100C isn't enough. The heat of evaporation has to be added also. The water at the bottom of the pot is at 100C; the water boils as heat continues to be pumped into the pot.

 

To answer the original question, the water at the bottom is hotter. As D_H mentioned the pressure and therefore the boiling point is also higher at the bottom. What happens is that water is heated at the bottom. As it is heated it becomes less dense and rises by convection. As it rises the pressure and the boiling point drop until the water actually boils. At that point some of the water is turned into water vapor, removing enough heat to keep the water at the boiling temperature.

Of course, as a rolling boil is established the whole liquid becomes turbulent and this analysis becomes much less realistic. The temperature variations become quite small and nearly uniformly distributed from top to bottom. At that point you can turn the heat down to the minimum required to maintain the rolling boil without really changing anything.

-Dale

 

I think the boiling happens at the bottom of the pot. The temperature of the bottom of the pot itself (not the water) is greater than 100C. This creates regions of superheated water that burst into waper vapor bubbles. Bring a pot of water to a very slow boil so that small vapor bubbles form at the bottom. By setting the heat just right, the vapor bubbles will disappear before they reach the top of the pot.

<i>Edited to add</i>
This is assuming a normal-sized household pot. The water at the bottom of the pot cannot boil if the pot is more than 2.1 km tall or so because the pressure will then exceed the critical pressure (rule of thumb: pressure increases by one atmosphere for every 10 meters of water depth).

 

Hmm, you are probably right. That would depend on the heat transfer between the water and the pot, which is undoubtedly not perfect, particularly during a rolling boil.

In any case (regardless if the boiling happens at the bottom or middle), the bottom is hotter.

-Dale

 

Why would the bottom of the pot burn, rather than the top?

Food stuff that touches the bottom of the pot loses its water content and then burns. The metal of the pot is far hotter than the water.

 

Isnt it amazing that when the gas Flame is on full mode
things get cooked faster, even though the temperature is always 100%

I smell something fishy,

OH I think my fish is burning on gas, I forgot !

 

There is a difference between temperature and heat. Stick a metal spike thru a potato before placing it in the oven to bake, and it will get done faster. You haven't raised the temperature of the oven, but the metal spike does transfer heat to the potato's center faster. Similarly, the air molecules in the very rarified atmosphere of Earth orbital space have a high temperature; they are moving quite fast to be at that altitude. However, the heat up there is low, because the air is so thin it's considered a vacuum; therefore there aren't many air molecules in contact with a satellite, so they don't transfer much heat to it.

How does this apply to cooking things on a stovetop? Well, for one thing, it's incorrect to say turning up the burner under a boiling pot increases the temperature of the bottom of the pot. So long as there's water in the pot, the pot can't get any hotter than the water-- 100 C at maximum. But turn the burner up, and the heat the burner transfers to the bottom of the pot goes up, and therefore the heat the bottom of the pot transfers to the water goes up.

As has been noted, food sticking to the bottom of the pot can drive out water in spots and allow the temperature at that spot to be raised past the boiling point, burning the food.

~~~~~~~~~~~

When you add food to a barely-boiling pot, the cold/tepid food cools the water it's in contact with. If the pot is barely boiling, convection currents will only slowly stir up the colder and hotter water. Since some of the water in contact with the food is therefore at less than boiling temperature, less heat will be transferred to the food.

Turn up the heat to a furious boil, and two things happen: One, the water is violently stirred up, so the colder water is no longer kept in contact with the food, but distributed thruout the pot. Two, the bottom of the pot transfers more heat to the water, and thus the water coming into contact with the bottom is more quickly raised back to boiling. Thus the average temperature of the entire pot-- and therefore the water in contact with the food-- is more quickly raised back to boiling.

After the food is heated thru to boiling temperature, you may as well turn the fire back down. Which is precisely why many cooking instructions call for heating something to boiling, then reducing the heat to a simmer. After the food is heated thru, furious boiling will only boil off the water and burn whatever comes into contact with the bottom, not cook the food faster.

And yes, for cooking pasta it's advisable to keep the water boiling furiously to help keep it from sticking together, but that's a separate issue.

I don't think you'd be able to measure the increase in the boiling point caused by the miniscule pressure at the depth of the water in a normal cooking pan, and it certainly won't measurably affect the cooking time. However, at a high altitude water does boil at a lower temperature, increasing required cooking time; you may have seen "high altitude cooking instructions" for that reason.

It is possible to raise the boiling point for cooking food by using a pressure cooker. Those seem to have fallen out of fashion, but when I was a kid ('60s) my mother frequently cooked chickens in a pressure cooker, requiring appreciably less cooking time.

 

When I boil water or soup or whatever, the boiling starts to happen at the top sides of the pot, not the bottom. DaleSpam's analysis is correct. The liquid is hottest nearest the source of the heat.

 

How hot can steam become? I dunno tell me somethings about steam.