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I don't have a kettle, so I use a cooking pot at home to boil water. I boil water on high in the cooking pot and as soon as I see the big bubbles/steam forming, I assume the water has reached 100° C. Is that correct?

If the water has reached 100° C and I let it settle off the stove for 1 min, what's the average temperature of the water after that period of time?

EDIT

I'm trying to make some coffee from my french press and from what I've read, people recommend to wait 1 min before pouring the hot water in the press. I'm not getting a lot of coffee flavour from the french press after letting it infuse for 5 min. I was curious to know if the temperature of the hot water can drop a lot in 1 min.

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You are correct with the assumption that the water reaches 100°C when it starts to boil. As for the average temperature, I think it would be really hard to calculate since you would have to take into account what temperature the room is in and what not. – duchessofstokesay Feb 12 '11 at 14:25
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Pick up the book Thermodynamics for Dummmies. It will help you calculate heat loss based on ambient room temperature and the size of the vessel. – Brian Feb 12 '11 at 16:59
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I'm gonna be a bit pedantic and say, "Use a thermometer". It's really the only way to tell with 100% certainty (limited by the acccuracy of your thermometer, of course). – Marti Feb 12 '11 at 22:54
    
At the Engineering Toolbox: Heat Loss from Open Water Tanks: engineeringtoolbox.com/heat-loss-open-water-tanks-d_286.html – Wayfaring Stranger Mar 21 at 20:02
up vote 6 down vote accepted

As long as you are talking about a normal pot with or without a normal lid (i.e. not a pressure cooker) and you are reasonably close to sea level, you're right, boiling water is at 100°C. However, if you start to climb in altitude, that is no longer the case, at 300m, water boils at 99°C, at 600m, 98°C and so on. Wikipedia has a page with information about High altitude cooking that contains a reference table.

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... and that it's mostly pure water -- if it's a solution (ie, there's salt or sugar disolved in it), the boiling point is slightly higher. (not much though, you can only raise it about °4C, and that's for a saturated solution, which would be very salty) – Joe Feb 12 '11 at 21:11
    
This would go for water in a kettle also though. – vwiggins Feb 14 '11 at 10:13
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This doesn't answer the important part of the question, which is how fast the water will cool. – Jefromi Mar 21 at 20:04
    
@Jefromi No, but that was not the original formulation of the question (the one I answered back then). – PaulRein Mar 31 at 9:04
    
@PaulRein It wasn't in the title, but it was the whole second paragraph of the body. – Jefromi Mar 31 at 14:20

Boiling fresh water is indeed 100c or 212f at sea level. However your question is a very good one. If all the water in your pot were boiling, it would all vaporize very quickly.

The water just above the hot spots inside the tea pot are just under boiling, and reach boiling just as they vaporize, but the water convecting around the edges is much cooler.

In general, it is usually considered that if you bring tea water just to boiling point (the whistle just starts blowing), and you pour a cup of tea that water temperature overall is closer to 180F or 82C which is the perfect temperature for steeping tea leaves.

Coffee steeps faster in hot water as well. In fact you can make an appreciably better cup of coffee starting with hot water rather than cold water in a basic drip coffee maker. The hot water runs through the system faster which means less of the harsher elements of the coffee is extracted from the bean. Hot water equals swifter extraction which equates to better quality, where as just like with tea, If you steep too long, the tea loses its fresh quality and becomes astringent.

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Tip: in your current phrasing, your question seems like a rather abstract physics question. You could get more informative answers if you expanded it to let us know what you are trying to prepare at a certain water temperature. Is it tea perhaps?

But to try to answer the first part of your question as stated: the Rouxbe cooking school has a video lesson demonstrating how you can identify different water temperatures without using a thermometer. For example, for the poaching cooking method (which is done in water at 71 to 85 degrees Celcius) you should look for the first small bubbles at the bottom of the pot and the first signs of steam from the surface. So assuming that the water is at 100 degrees Celsius as soon as you see steam forming is not necessarily correct. If you heat up the water further than the poaching temperature range, you get at the temperatures for simmering and gentle boiling. For a vigorous boil (100 degrees Celcius, which is the maximum temperature that water can reach at sea level) you have to wait until the water is moving and steaming faster, with big bubbles appearing on the surface.

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The cooling rate will also depend on the mass (volume) of the water, the mass of the pot, the thermal transfer capacity of the pot and anything it contacts, ambient temperature, air pressure, humidity, purity of the water, etc. The answer to your question is "close enough".

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