The 'hot water freezes faster than cold water' controversy

[ripcord]

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.

 

[sp123]

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

 

[frnash]

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 An additional amount of energy, at the rate of 79.72 calories/gram (the energy of the heat of fusion, must be withdrawn from that water to convert it to ice (perhaps ice crystals), still at 0°C, or 32°F — the liquid must turn to solid before the temperature can continue to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

 

[700xcsp]

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.

 

[frnash]

<font color="0000ff">700xcsp:
"Did the same thing with room temp water … and cold water."</font>

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

So perhaps with the hot water &#0151; by definition at a higher energy level &#0151; 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.

 

[thebluff]

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

 

[cmharcou]

send mythbusters and e-mail.

 

[soopy]

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. ? .

 

[admin]

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!

 

[swanker]

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

swanker

 

[frnash]

<font color="0000ff">"Too cool."</font>
Yeh, -25°F, I'd say that's the understatement of the week!

 

[ubee]

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!

 

[ripcord]

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)

 

[lvr1000]

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.

 

[lenny]

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

 

[frnash]

<font color="0000ff">lenny:
"I put (2) 16 oz plastic cups outside …"</font>

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!

 

[dfattack]

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.

 

[redrev]

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.