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A question about probably the most mundane subject in cooking: boiling water.

For cooking techniques where you drop ingredients in simmering or boiling water - such as for vegetables, pasta, many rice recipes - I have often seen the recommendation that you start by putting cold water into a pot, then bring it to a boil. Why would you not start with hot water from the tap? It's going to be quicker than heating cold water, and your water heater is going to be way more energy efficient than your stove top at heating the stuff.

In particular, is there any physical or chemical process that starting from cold water encourages or prevents from happening?

(To reiterate: in the case where you add stuff to cold water and then start heating it, there clearly is a difference with starting with hot water; this question is about the case where you drop your ingredients in pure water that's already boiling.)

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Cold water boiling faster is one of those persistent myths that proves that many people are unable to think critically. Feel free to laugh at people who haven't thought it through. :-p –  ceejayoz Oct 24 '10 at 20:52
    
Have you considered performing a simple test measuring how long it takes your cold water to boil versus warm water? –  Zoredache Nov 13 '10 at 4:57
    
I always start from hot water. But then I have a newer hot water heater that I know is free of sediment. –  renegade May 19 '11 at 20:04

6 Answers 6

up vote 19 down vote accepted

Some people say cold water boils faster than hot water, this is false, found here and here.

One reason might be (from the first link): "Some water heaters may introduce additional sediment into the water, giving you another reason to consider starting with cold—at least, if time is not of the essence."

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I haven't read all the links, but what IS true is that if you start with boiling water and make ice out of it, you'll get ice faster than starting with cold water. But you'll get LESS ice from the boiling than the cold water - it's faster because some evaporates. –  Ward Oct 25 '10 at 2:19
    
The idea that cold water boils faster than hot is total nonsense. If you try to boil cold water, it will take some time to reach the temperature of the hot water. From that point, it will take exactly the same amount of additional time to boil as the hot water would have done if you'd started with it. –  David Richerby Aug 14 at 17:46

There have been plumbing systems in which the hot water was likely to have dissolved more [toxic|unsightly|unpleasant-tasting] material from the pipe walls or joints than the cold. In particular any system that uses lead-based solder, can leach minute{*}, but detectable amounts of lead into the drinking water, and the hot water is more efficient at this.

In this case that advice amounts to "use the clean water".

{*} Really minute. Like "Use this water all you life and not suffer any ill-effects" minute. But it can be detected, and who wants to chug down a glass of lead solution...

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If a bear chugs a glass of lead solution while alone in the woods and doesn't suffer any ill-effects, ... –  Roger Pate Oct 25 '10 at 1:53

My hot water tap is supplied by a combi boiler which heats the water on-demand, supplied by the same cold water source as my cold water tap. As a result, I'm confident that the water is reasonably fresh and clean. I don't use it for brewing tea or coffee, but I'm happy to boil vegetables and rice in it. It saves a couple of minutes bringing the water to the boil.

My parent's hot tap is supplied from an insulated immersion heater tank. The same water can sit in there for days; it may heat and cool several times in that period. It is supplied by a header cistern in the attic. Last time I looked at the header cistern, there was a crop of dead flies floating on the surface, and some unidentifiable gunge settled at the bottom. This hot water is suitable for bathing and cleaning; it's not suitable for cooking.

If you don't know the details of your plumbing; and you're not sure it's safe; don't cook with water from your hot tap.

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Vegetables and rice? You are supposed to put grains in cold water and bring to boil slowly, because they profit from the soak. I can understand starting with hot clean water for recipes where you add the ingredients to boiling water, like pasta and short-cooking veggies, but not for rice. –  rumtscho Dec 15 '11 at 12:29
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My basmati recipe is 1 part rice, 2 parts boiling water, salt, simmer covered for 10-12 minutes until absorbed. Brown rice might benefit from a soak though. –  slim Dec 16 '11 at 13:04

There has been a lot of discussion over the years in cooking circles about whether cold water or hot water comes to a boil faster, and the people that wrote those recipes are passing on their determination on to you. There is zero functional difference between one pot of boiling water and another. Once a roiling boil has been achieved, the water will always be the exact same temperature. That is one of the reasons that we use water as a cooking medium.

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Assuming the two pots of boiling water are at the same altitude, of course. –  Michael Hoffman Dec 15 '11 at 21:36
    
assuming pressure to close all the loopholes. –  sarge_smith Dec 16 '11 at 1:17

No, there are no different physical processes. The only thing to be wary of is that water heaters can be pretty icky places so, depending on your system, you might want to avoid drinking that water. On the other hand, if you have an electric kettle then by all means use that to heat the water: it's clean and, like your water heater, much more efficient than heating water on the stove.

The idea that cold water boils faster than hot water is complete and utter nonsense. Suppose you have a pan containing a litre of water at 40°C. That pan will take some amount of time to boil – let's say five minutes. Water has no "memory" of its past. It does not know how it got to 40°C so how it got to 40°C cannot possibly influence how long it takes to boil from that point. In other words, any litre of water at 40°C will take five minutes to boil in that pan. In particular, if you put a litre of water at 20°C in the pan and start heating it, it will take some amount of time to come to 40°C. After that, it will take the standard five minutes to boil. In other words, surprise!, cold water takes longer to boil than hot water because, first, the cold water has to be turned into hot water and then the hot water has to be boiled.

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well this question seems to be quite old now but there is another very important reason why you should start with cold water when cooking related to the physical and chemical properties of water that did not appear to make it into any of the other answers.

The answer is related to the reason that warm pop gets flat. Compared with warm water, cold water's ability to store dissolved gases is superior. Warm water on the other hand tends to expel dissolved gases (hence the reason cold pop stays fizzy longer than warm pop). The reason you want to keep pop cold is to keep in all the CO2, but there are other gases dissolved in water that you generally want too, like oxygen gas (O2).

Hot water has probably been sitting in the plumbing system for some time, so a great deal of the oxygen gas has probably been expelled. On the other hand, cold water from the taps is more "fresh" and enriched with oxygen gas. Even after boiling water, the trapped gases will take some time before they escape. So if you start with cold water, whatever you're cooking with the water will become more enriched with oxygen gas than if you started with warm water.

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What foods are enhanced by dissolved oxygen? And in what way? –  slim Dec 15 '11 at 11:05
    
I found that water has been sitting in a kettle for a long time and is reused it seems to have a "dryer" taste than water that is fresh from the tap, but maybe that's just me. Dissolved oxygen is important to have in water though; if you were to drop a fish into a tank with all the oxygen gas sucked out, the fish would die. But alas I am not a biologist or an experienced chef, I'm an engineer. But after a bit of work I did find this interesting article on dissolved oxygen: freedrinkingwater.com/water_quality/quality1/… –  Bizorke Dec 15 '11 at 21:37
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This is incorrect. Boiling water has almost zero dissolved gas in it so, from that point of view, it doesn't matter whether the water started hot or cold. –  David Richerby Aug 14 at 17:30

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