Why Does Hot Water Freeze Faster?

[Admiral Valdemar]

So why does this occur specifically in bird baths or frosted car windows? In have heard several theories such as less oxygen dissolved in the hot water to an unknown environmental factor, but they don't tell the whole picture.

Comments.

[kojikun]

uh.. it doesnt. go boil a pot of water and stick it in the freezer alongside a cup of tap water.

[Zoink]

So why does this occur specifically in bird baths or frosted car windows? In have heard several theories such as less oxygen dissolved in the hot water to an unknown environmental factor, but they don't tell the whole picture.

Comments.

Evaporation of water carries extra heat away with it, the heat of evaporation. The hot water will supposidly catch up to the colder water quickly, then with less volume (and heat energy) will lose its heat first and freeze.

I vaguely remember a science show for kids I watched a really long time ago, where this would be dependant on the humidity of the air, and might not work in your freezer, but supposidly works outside. I have never tried it myself though.

[Admiral Valdemar]

uh.. it doesnt. go boil a pot of water and stick it in the freezer alongside a cup of tap water.

Considering I put hot water in the bird bath today and is was frozen long before an equal amount of cold water was the same day, I don't think so. Plus scientists have applied Newton's laws of cooling to it which fit partly but break apart in some areas.

So why does this occur specifically in bird baths or frosted car windows? In have heard several theories such as less oxygen dissolved in the hot water to an unknown environmental factor, but they don't tell the whole picture.

Comments.

Evaporation of water carries extra heat away with it, the heat of evaporation. The hot water will supposidly catch up to the colder water quickly, then with less volume (and heat energy) will lose its heat first and freeze.

I vaguely remember a science show for kids I watched a really long time ago, where this would be dependant on the humidity of the air, and might not work in your freezer, but supposidly works outside. I have never tried it myself though.

Well that was another possible solution, the fact is that no real scientist is researching it since they have bigger problems to solve. I still don't have any full answers on this, must keep searching.

[Darth Wong]

If true, this would be situationally dependent and probably has something to do with thermal conductivity. Obviously, once hot water "catches up" to cold water, there's no intrinsic reason why it should race ahead. So a couple of reasons pop up:

1) Temperature gradients: the outside of the cold water freezes faster, and the ice layer acts as an insulator. The hot water has more active convection and resists freezing longer, so that its average temperature is lower when the outside finally DOES freeze.

2) If it's hot enough, a lot of it evapourates quickly, so there's less water remaining to freeze.

Just a couple of ideas, and maybe there's something else (assuming this phenomenon is real); I've thought of helping my boy do a science project on this when he gets a little older. The trick is to make sure the water is actually frozen all the way through, instead of just being frozen on the surface. Ideally you'd want monitoring of the centre temp, perhaps with a thermocouple.

[Enlightenment]

Hot water evaporates faster than cold water. Evaporative cooling is highly effective. Connect the dots.

[Lord Edam]

So why does this occur specifically in bird baths or frosted car windows? In have heard several theories such as less oxygen dissolved in the hot water to an unknown environmental factor, but they don't tell the whole picture.

Comments.

Originally discovered by an African high school student called Mpemba in 1969, this is known as the Mpemba Effect.

linkage
(site found using www.physics.org)[/url]

[Durandal]

Don't forget that various types of water (tap vs. pool water) have different minerals in them, which affect the critical temperature points.

[ArthurDent]

Use distilled water to resolve that issue.

[aerius]

I remember trying to figure this thing out when I was a kid. I'd always wondered why they used hot water for ice rink zambonies, and figured that it had something to do with freezing speed. I attempted to verify this by freezing ice cubes in my home freezer, we had 2 identical ice cube trays and I'd fill one with hot and the other with cold water. With our freezer the cold water always froze solid first, so I was at a loss to explain the ice rink zambonies until I saw a TV show years later. It turns out that they use hot water because it melts the ice and smooths it better giving a better surface not because it freezes faster.

[Darth Wong]

According to the link Edam provided, it only happens under certain conditions. The discoverer was using wooden buckets; wood is a good insulator, so the cooling occurs primarily through the top.

In many other conditions, you probably won't see this effect, since (naturally) any effect which causes hot water to be cooled faster (such as evapouration) will diminish as it approaches the temperature of the cold water.

In an insulated container with only a small proportion of the water being effectively cooled, internal fluid mechanics probably play a greater role.

[Admiral Valdemar]

Ah, Edam had the link to the anomaly that the paper described today (where I got this idea). I forget to mention the name of this effect, it seems to be apparent in winter when clearing car windscreens and filling bird baths, I had my doubts too but it seems real enough, the type of water and way it evaporates were the main reasons I could think of without burning my tired brain out.

Cheers for the answers!

[HemlockGrey]

This is all heresy. It is the work of God.

[SeebianWurm]

Shut thine mouth, blasphemer. God does not move in mysterious ways, he moves in U-Hauls.

[Beowulf]

This is all heresy. It is the work of God.

Blasphemer!

[Warspite]

It's true, and besides Mpemba, there are also references in Sir Francis Bacon and Aristotles.
There are four factors that influence this phenomenum:
Evaporation: explained in earlier posts, evaporation carries latent heat away, and diminishes the amount of mass of water to freeze.
Convection: when the superficial water gets cooler, it's density increases, sinking, whilst warmer water, less dense, rises. (If the initial temperature would be at 4ºC (highest density), this phenomenum wouldn't be clearly observed.)
Dissolution: water contains dissolved gases, that lower the point of solidification, by heating the water, most of these gases are eliminated, hence freezing will occur at a higher temperature, than with colder water.
Superfreezing: this phenomenum is related to the existance of impurities that act as seeds for the formation of ice cristals, whilst the temperature is lower than freezing point, where it's possible to mantain a liquid state, with a low enough temperature, this cristals can propagate through the whole liquid very fast, raising the temperature back to 0ºC. (granted, this effect may not be very influent in the whole phenomenum.)