Thread: Can hot water freeze faster than cold water?

Here is an interesting topic that defies logic.

http://www.math.ucr.edu/home/baez/ph...hot_water.html

I for one growing up in a very cold climate always remember the hot water pipe freezing and the cold pipes not. The above URL has some interesting info.

They are calling this the "Mpemba Effect" .

I heard that you should always fill the ice cube trays with hot water and they'll freeze faster, and I didn't believe it until now.

sounds(or rather look's) interesting,

But why is that? WWhat makes it happen, or is it in the site?

it ahsn't been proved yet! but it is more than likely true,

i read it, tis very interesting to read about it, a little too iteresting!

I guess Ill have to read it.

I have heard several theories for this phenomenon. One is that hot water evaporates faster and so there is less water to get to freezing point than in the cooler water. I have also heard that it's because the warmer water will cool faster initially and that inertia will keep the rapid rate of cooling going until it is frozen, thereby freezing before the cooler water.

Let me explain. Let's say there are two cars, they are side by side and they are both trying to get to a finish line. One car starts heading towards the finish line, but at a slow pace because it is closer. The other car is farther away from the finish line, but starts off going faster. Neither car accelerates as it nears the finish line, they just roll. The car farther away is obviously rolling faster, and the car closer is rolling slower. Obviously the car farther away is the warmer bucket of water, and the one closer is the cooler bucket of water. IN SOME CASES (that is key), the car that started farther away (aka the warmer water) will have enough inertia to overtake the slower moving car and get to the finish line faster, thereby winning the race (aka freezing first).

Ok, I know that's really confusing, but do you understand what I am trying to say? And I'm not saying that's right, that's just a theory.

The trouble with that theory is there is no "inertia" involved with the reduction of a temperature, and it's difficult to see what the analogous effect might be. What is happening in fact is that the molecules are moving more and more slowly.

I don't know if boiling the water makes it freeze faster, but it has the effect of removing the air that is dissolved in normal water, which is why "boiled" ice cubes are clear while normal ones are cloudy. Perhaps it is this that has the effect on the time taken for freezing to occur.


EDIT: I've changed my mind, the analogy of "inertia" is the temperature gradient, obviously, which could be thermodyanically proved to be much greater in respect of the higher temperature (though the specifics of the maths is beyond me).

I can think of one explaination.

In cold water that is freezing all the energy is basically being passed off to the surrounding air.

In hot water that is freezing you have water evaporating and taking some of the energy with it... basically increasing the rate.

My only problem is that I didn't think it would increase the rate enough to make up for the initial higher temperature. At some point it will be back at the same state as the cool water... but at a later time.

I would actually expect it has something to do with molecular density. The spacing between molecules somehow allowing slower moving cooler molecules to penetrate deeper into the water faster.

Think of it as a crowd of people moving in a shopping mall, if they are somewhat spaced apart "Hot" and run into a crowd moving the opposite direction "Cold" that new crowd will penetrate and mix in with ease, the end result slowing both groups. As more people are introduced into the ever growing crowd from multiple directions the crowd stops moving and they freeze.

Now in the case of the "Cold" crowd that is more densely packed when the two crowds meet up it's more difficult for the two to mingle. The end result is "Cold" crowd meeting the other "Cold" crowd is not slowed as quickly as the "Hot" crowd with all that extra space in between meeting the "Cold" crowd.

So in a sense the cold runs off the more solid material and penetrates the hotter material with all of it's holes.

Just a wild guess.

Hey everyone,
I know it's fun making crazy speculations... but why don't you first read the paper linked in the first post, and find out what others have already thought about and tested?

Originally Posted by Pete

Hey everyone,
I know it's fun making crazy speculations... but why don't you first read the paper linked in the first post, and find out what others have already thought about and tested?

Umm.....we could do that

Of course that would not be as much fun. Now I have to go find out...

Edit:

Ok, I read the thing...and they don't even know why. Ok...I like my theory

Hope this helps! Water cooling goes to 0 degrees centigrade then rebounds back up to 4 degrees then arranges itself in a crystaline pattern then freezes. This is to do with the fact that ice is less dense then water and there is less potential energy in the crystalline arrangement. Just look at a pond frozen over in winter the top freezes but the bottom level remains unfrozen even though the bottom is colder. It can't absorb energy to reach 4 degrees centigrade

Sweet! Now, has anyone tried it? Go get two ice trays, fill one with cold water, and one with hot water, and see which one freezes first, checking every 5 minutes or so. What did you discover?

Here is my speculation on why hot water would freeze faster:

How quickly a liquid cools depends on the temperature difference between the liquid and its surroundings, so a hot liquid will initially cool more quickly than a cold liquid. However, the rate of cooling for the hot liquid will decrease as it cools, since the temperature difference between the liquid and its surroundings is decreasing. So, although a hot liquid will initially cool faster, eventually it will reach the same temperature as the colder liquid and then they should both cool at the same rate, since they will have the same temperature difference.

However, the hotter liquid will also be evaporating as it cools. So, once the two liquids reach the same temperature and begin cooling at the same rate, the liquid that was initially hot will now have less mass to cool due to its evaporation; since there’s less water there to freeze, it will freeze faster. The easiest way to check this would probably be to weigh them and see if the hot-water ice cubes end up weighing less than the cold-water ice cubes.

Originally Posted by phephiphophum

Hope this helps! Water cooling goes to 0 degrees centigrade then rebounds back up to 4 degrees then arranges itself in a crystaline pattern then freezes. This is to do with the fact that ice is less dense then water and there is less potential energy in the crystalline arrangement. Just look at a pond frozen over in winter the top freezes but the bottom level remains unfrozen even though the bottom is colder. It can't absorb energy to reach 4 degrees centigrade

I don't think that's true. Water is most dense at 4C because that's as cold as it can get without expanding to form a hydrogen-bonded lattice, but there isn't any temperature change associated with it.

tell me why then that only the top of the pond freezes

Water starts becoming less dense below 4C as it starts to form a lattice from hydrogen bonding. That causes the coldest water to float to the top. Once it freezes, it insulates the layers below it and keeps them from freezing. Also since the water freezes from the top down, the heat of fusion that's released when more water freezes causes the rest of the water to warm up, which slows freezing.