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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
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3 Answers

up vote 7 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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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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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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