The 'hot water freezes faster than cold water' controversy

General Discussions: ASK John: The 'hot water freezes faster than cold water' controversy

By ripcord on Friday, January 16, 2009 - 11:37 pm: Edit Post

John, can you provide a simple, understandable explanation as to how & why, and under what conditions, it's 'sometimes' possible for hot water to freeze faster than cold water?

I know it's possible to test this theory and come up with results both ways, but I can't find a 'simple' explanation as to what variables it takes to get which results.


By sp123 on Saturday, January 17, 2009 - 01:45 pm: Edit Post

well because the molecules in hot water are moving faster than cold water and when you drop the temputure so fast it causes all them to stop so quickly thus they all freeze up. some of the reason why


By frnash on Saturday, January 17, 2009 - 03:01 pm: Edit Post

I am really having difficulty accepting this theory!

For an example, let's say we are introducing a quantity of water at an initial temperature somewhere near the boiling point into an environment at an ambient temperature of -40°C (which is conveniently equal to -40°F).

1. A certain amount of energy, at the rate of 1 calorie per gram, (the specific heat of water/ice) will have to be removed from that water just to reduce its temperature to the freezing point (0°C, or 32°F):

2. An additional amount of energy, at the rate of 79.72 calories/gram (the energy of the heat of fusion, must be withdrawn from that water to convert it to ice (perhaps ice crystals), still at 0°C, or 32°F — the liquid must turn to solid before the temperature can continue to fall.

3. Of course, still more energy (the specific heat of water/ice) will have to be removed from the resulting quantity of ice to reduce its temperature from the freezing point to ambient (say -40°C, conveniently = -40°F, as noted above)

The only variant in the above calculations is the initial temperature of the water.

Clearly the warmer the water, the more energy it will have to lose in the process.

Thus if it takes any amount of time at all for the energy loss to occur, it is quite impossible for hot water to freeze faster than cold water!

Of course for relatively small quantities of water and such a low ambient temperature that incremental energy loss can probably occur in an instant, regardless of the initial temperature of the water, especially if you toss a cup of water up into the air (as suggested in an experiment described elsewhere in this forum — I note that in that case they did not try the experiment with the same quantity of water at an initial temperature close to the freezing point!), thus scattering the water as droplets, thereby increasing the surface area on which that heat transfer will occur!

Or is there something I'm missing?


By 700xcsp on Saturday, January 17, 2009 - 03:05 pm: Edit Post

It was -23 here Thursday and at work we were talking about this. We took a cup of steaming hot water and tossed it outside in the air and it evaporated. Did the same thing with room temp water and it froze when it hit the ground and the same thing happened with cold water. Just thought I would tell you our little experiment.


By frnash on Saturday, January 17, 2009 - 03:28 pm: Edit Post

700xcsp:
"Did the same thing with room temp water … and cold water."


So the hot water evaporated?… and the other two did not?

So perhaps with the hot water — by definition at a higher energy level — all those high energy li'l water molecules "flew apart" more quickly than in the case of the more "lethargic" cold water molecules, thus increasing the aggregate surface area of the hot water and allowing the heat transfer noted in step 1 of my above note to occur more rapidly?

Seems a bit of a stretch to me. Ya never know; "thermodamnamics", not a favorite subject. :-)


By thebluff on Sunday, January 18, 2009 - 06:30 pm: Edit Post

I know on our boiler heat, the hot water will freeze very quickly when household temp water lines do not freeze. my dad is a former heat guy, says that is a true-ism


By cmharcou on Monday, January 19, 2009 - 07:25 am: Edit Post

send mythbusters and e-mail.


By soopy on Monday, January 19, 2009 - 08:24 am: Edit Post

someone once told me it was due to the condensation that develops on the exterior of hot water pipes in cold weather. Thus the quicker- to- freeze theory. ? .


By admin on Monday, January 19, 2009 - 02:15 pm: Edit Post

I did a little internet research on this topic and it seems as though it may be more than just a myth. However, there are some parameters that need to be met as well.

First, we are not talking about a volume of water that is tossed into the air, the sites I checked out were all talking about a volume of water within a container and also did not talk about the entire container freezing, just the top of the water that was exposed to the cold air.

There are a number of factors that scientists think may be at work, like the fact that hot water will evaporate quicker and evaporation takes a lot of enery and that energy loss can cause the water at the surface to cool faster. Some issues with vapor pressures too and a few other factors were mentioned, but no sound and tested fact as to why the container of hot water sometimes froze faster than a similarly sized container of cool water.

Sounds like a good one for Mythbusters to me!

-John

PS. Soopy- Who ever told you that condensation will form on a hot water pipe is sure smoking some good stuff. Condensation happens when air encounters an object that is cold enough for the moisture in the air to condense on it. A warm pipe would cause just the opposite effect!


By swanker on Monday, January 19, 2009 - 08:16 pm: Edit Post

John check out the video with Adam from the Skyview Lodge a cup of hot water and minus 25. Too cool.

swanker


By frnash on Monday, January 19, 2009 - 08:50 pm: Edit Post

"Too cool."
Yeh, -25°F, I'd say that's the understatement of the week! :-)


By ubee on Tuesday, January 20, 2009 - 01:03 pm: Edit Post

any plumbers out there? when ever i have gone to fix frozen pipes 99% of the time it was a hot water pipe! other 1% was cold water only! no explanation just observation!


By ripcord on Tuesday, January 20, 2009 - 07:37 pm: Edit Post

Thanks John, I did some internet research myself and found out even among the scientific community there's no consensus on a conclusive explanation for this phenomenon. As you stated there are several parameters that need to be met. My own observations show that, while a bucket of hot water will form a 'layer' of ice first, the bucket of cold will freeze 'solid' sooner... so the definition of 'freezes first' is what raises controversy.

I checked 'Mythbusters' website and apparently they tried this experiment using ice cube trays... their conclusion, 'if you want to make ice cubes quiclky in your freezer, use cold water.

As for pipes freezing, this seems to have more to do with 'supercooling' (where water remains liquid when it's temperature is below 32 F.), something to do with the molecules not being able to arrange themselves to form ice.(That's a whole nother topic)


By lvr1000 on Tuesday, January 20, 2009 - 08:43 pm: Edit Post

I always thought that the hot water pipe usually froze first because hot water is used less in a house than cold, toliets and water softner the main reason. Therefore sits longer in the pipe before moving. And if you think about it, the water is only hot right after it comes out of the heater. Once it sits, it is the same temp as the cold.


By lenny on Saturday, January 31, 2009 - 12:47 am: Edit Post

I did a small test, I put (2) 16 oz plastic cups outside, 1 with boiling water and the other cold tap, the cold froze considerable faster than the boiling cup. it's a myth IMHO


By frnash on Saturday, January 31, 2009 - 02:21 pm: Edit Post

lenny:
"I put (2) 16 oz plastic cups outside …"


As expected for "still" water; curious to hear if it's any different if you toss each cupful of water up in the air … particularly on a very cold day, preferably at least in the minus teens. Maybe a bit late for such temps this season? Certainly no chance of such temps during the next week!


By dfattack on Sunday, February 08, 2009 - 10:08 am: Edit Post

It's what "lvr1000" said. In a house the hot water lines are used less, therefore they freeze first. Not sure if "ripcord" was referring to household pipes or not, but that is the answer.


By redrev on Tuesday, February 17, 2009 - 04:34 am: Edit Post

an old theory is that hot water freezes quicker but i think the hot water lines get used less. you used to see a lot of ice makers hooked to hot water so everyone thought it froze quicker. actually they hooked to hot water to make the ice appear more clear.


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