does cold water boil faster than pre-heated water in a pot on the stove?

hodag...

Thanks for your question.  I have often wondered the same thing and I
even tend to wait for the water to warm up coming out of the spigot
before putting in the pot to boil.

The boiling point of water is 212 degrees Farenheit (100 degrees
Celsius).  In order for that temperature to be attained, we apply heat
to the water.  Being that I am American, all temperature references
are in Farenheit.  If the water is 65 degrees, it must increase
roughly 150 degress to reach the boiling point.  However, if the water
is 80 degrees, it need only increase the temperature by roughly 130
degrees, or 20 degrees less, to attain the boiling point.  Another
analogy would be to consider ice.  Ice is nothing more than water that
has reached a freezing point, or 32 degrees Farneheit, or lower. So,
it is simply at a lower temperature.  If you take a cup of water that
has frozen, a cup of water at room temperature, and a cup of water
that is warmer than room temperature, the warm water will come to a
boil quicker than than the room temperature water which will come to a
boil quicker than than the frozen water.

From an energy standpoint, however, it is more cost effective to take
the water from the tap and fill your pot as opposed to waiting for it
warm up as it takes more energy for your hot water heater to heat up
water than it does for your stove.  So, while warm water may boil
quicker, it is more energy efficient to boil water at the temperature
it comes from the tap.

One way to decrease the time to boil is to cover the pot with a lid.

Thank you for your question and I hope the information I have provided
was helpful.  If you need any additional clarification, please let me
know.

Regards,

-THV

Search Strategy:
hot water boils quicker than cold water

References:
Hot vs. Cold Water: Boiling Experiment
http://www.geocities.com/CapeCanaveral/8370/boil.htm

The Official Newsletter of Middle Georgia Electric Membership
Corporation
http://mgemc.com/oct_01/03.html

Dear Hodag (and Tar_heel_v),

Tar_heel_v's first sentence suggests that s/he has perhaps misread
your question.  If he is tempted to wait for warm water, he is
presumably expecting that to boil sooner, and he goes on to suggest
that that is the case.  But you (Hodag) may understand about energy
but may nevertheless be suggesting that it may be that the colder
water would boil first.  Whilst I don't believe that this will be the
case, it is worth considering such contrary-to-intuition
possibilities.  Indeed, if we reverse the problem, there is much
evidence that - in certain circumstances - warm water freezes sooner
than cold water.  See the (different) Usenet Physics FAQs at

http://www.weburbia.com/physics/hot_water.html

and

http://hepweb.rl.ac.uk/ppUK/PhysFAQ/hot_water.html

Incidentally, there are other problems with the answer.  The
suggestion that "[i]ce is nothing more than water that has reached a
freezing point, [...] it is simply at a lower temperature" ignores the
very significant concept of latent heat.  If ice were simply cold
water it would not be solid.  And every time snow thawed it would do
so instantaneously when the temperature rose, causing catastrophic
floods.  Vast quantities of polar ice would thaw and freeze by the day
and by the season, causing enormous waves and variations in sea level.

And why do we think that "it takes more energy for your hot water
heater to heat up water than it does for your stove"?  It will
certainly take more energy for your heater to heat an entire tankful
than for the stove to heat a potful, but that's cheating, isn't it?! 
There is every reason to suppose that the heater may do its job of the
first part of heating your potful of water more cheaply, because (1)
the water tank may be lagged, whereas the pot isn't, (2) less heat is
probably lost from the heat source itself in the heater than from the
stove, and (3) the energy source used by the heater may well be
cheaper than that used in the stove (where convenience is also an
issue).

I hope this helps.

Carnegie

You asked, "does cold water boil faster than pre-heated water in a pot
on the stove?", and you got lots of words in reply. If you were to ask
this question of an average kid, she would probably get two saucepans
and do a kitchen experiment, supervised by a parent. Sometimes kids
are just smarter than adults; adults over-intellectualize and
pontificate, and kids just go figure it out. So I suggest doing a
kitchen experiment. Get a parent to supervise if you're a kid.

As far as I know, using a modern, insulated water heater to heat a
given amount of water from cold-tap temperature to a typical
water-heater temperature takes MUCH less energy than a kitchen range
would use to do the same thing. But it is still not a good idea to use
hot tap water for cooking. Water drawn from the water heater can be
significantly lower in quality than cold tap water. See question 14
here:
  http://www.fcwa.org/water/faq.htm

and here's another source:
  http://www.drweil.com/app/cda/drw_cda.html-command=TodayQA-pt=Question-questionId=4078

While some of the critisism in the comments are justified and/or
helpful, tar heel did come to the correct conclusion. Warm water boils
sooner than cold water, water boils sooner than ice, and you typically
use more energy if you get the water from the hotwater tap. The
exceptions to the later would be a very small volumn of pipe between
the centeral hot water heater or a large volumn of water to be boiled
and/or the water from the tap was already hot due to resently drawing
hot water from the tap for some other use. The reason is the hot water
needs to heat the cold pipe enroute to the tap and hot water is left
in the pipe to get cold after you draw the desired amount of hot
water. Typically the better efficiency of the central heater is not
sufficient to overcome the extra losses, which do not occur when cold
water is used.
Carnegie and  weburbia suggestion that hot water might freeze freeze
before cold water occurs only under unusual conditions and the reverse
is even more rare(probably never). Carnegie; what do you mean by
"lagged"?
High route is correct; The hotwater tap supplies water of lower
quality than the cold water tap, with rare exceptions. Where the hot
water heater is set long term at low temperature and hot water is
rarely drawn from the centeral heater, the hot water tap may supply
dangerious water.
Covering the pot does reduce losses and typically the water reaches a
boil sooner.  Neil

Dear Neilzero,

By "lagged" I meant "insulated" (in the sense of surrounded by
lagging).  Sorry about that.

(I am fascinated to discover now that this word, though common in this
sense here in the UK - and apparently also in Ireland, Australasia,
and South Africa - seems not so in the US.)

Carnegie

Here's a question about rates of cooling that was asked in an
interview to get into Cambridge University once:

Is there any difference between the temperatures of the following two
cups of coffee: (1) Make black coffee black, wait for five minutes and
then add the milk (2) Make black coffee, add the milk and then wait
five minutes.

There are no other outside influences and the amount of coffee and
milk is the same in both cases.

I have not posted this as a question because I know the answer and
just wanted to stimulate further debate if people are interested.