Scientists offer £1,000 prize for answer to why hot water freeze faster than cold.

I was thinking more of the lines that the hot water would evaporate creating less water in the dish, glass, or whatever. So making less water to freeze. Where as the cold water is not evaporating as fast and has more to freeze.

Just a guess and thought it is a good one.

Originally posted by dnawrocki
I was thinking more of the lines that the hot water would evaporate creating less water in the dish, glass, or whatever. So making less water to freeze. Where as the cold water is not evaporating as fast and has more to freeze.

Just a guess and thought it is a good one.

Not so sure. By that reasoning a half glass of water would freeze before a full glass? Maybe, but that doesn't make sense if they are both in the same freezer at the same temperature.

reply to post by Bedlam


You do make a valid point with the solutes but that doesn't take into account the higher starting temperature. The boiled water will start to freeze at slightly higher temp, but this doesn't account for the more rapid loss of heat from a higher temperature. If I were to allow the boiled water to come to room temperature and then place it in the freezer it would freeze only marginally sooner. The still hot water will freeze much faster than room temp. Convection currents are responsible as they carry the heat away from the water at an accelerated pace allowing to lose more heat faster. Moving air is much more effecient at dissapating heat than stagnant air.

Originally posted by LogCog
reply to post by Bedlam

 

You do make a valid point with the solutes but that doesn't take into account the higher starting temperature.

By far, the bulk of the time spent in freezing takes place at the change of state. The drop from the liquid's ambient temperature to near-freezing is relatively quick.

reply to post by Bedlam

What would you say if instead of 100C the water was, say, 85C? 85C still > 35C.

I'm thinking that the molecules being much more excited when hot gives a heat convection transfer a boost using the kinetic energy of the much faster acting molecules to give it a speed boost in this natural cooling process as heat transfers..

It was the first thing that popped into my head anyways

reply to post by Bedlam


The fact is that both reasons are correct but there are differing opinions even today as to which factor in the cooling/freezing process is the most important. My guess is the currents and yours is gas dissolution. Let's get together and do some science!

A quick search on the subject found all kinds of interesting things. Wikipedia says:

The Mpemba effect is the observation that warmer water can sometimes freeze faster than colder water. Although the observation has been verified, there is no single scientific explanation for the effect.

Note that they say "can sometimes freeze faster". As has been stated above, the answer is partly due to convection which would be more pronounced in an enclosed freezer. Also, the definition of when it is considered to be frozen can affect the outcome (ie. do you stop the experiment when the water reaches 0 C. or when it freezes into a solid block?).

Another factor is whether frost forms on the top; cold water will develop frost more quickly and the frost tends to insulate the water lower down.

Found it all here

reply to post by Threegirls


Ice particles float, not sink; just look at the ice cubes in your next drink. Water is (as far as I'm aware) the only liquid that freezes from the top down. Which is rather fortunate for all the fish that live at the bottom of frozen lakes, right?

send me the money - this is easy to answer almost elementary,

120 degrees - is the temp of water
70 degrees - is the temp of the environment
30 degrees - is the freezer

now if you put 70 degree water in the freezer at the same time you put 120 degree water in the freezer - the hot water will freeze faster than the 70 degree water. why you ask... its easy - the hotter water it's energized more active - the active atoms of hot water can lose their radiation faster because the whole mass of water is in a hieghten state (lighter state) of liquidity - (its closer to 212 where it will change state into Steam). since the molecules are in motion they can lose their radation faster thus once the avenues / capularies are formed from the differing temperture clumps (molecular temperature of individual H2O molecules attract the same temp molecules) these capularies form from bottom to top that allows for faster conversion into a solid. research Hail, structures for an example of the capularies. or look close at an ice cube from hot water. regular luke warm water bearly forms the tube like structures.

can I have my check please...

Work = ΔE_k


Hot water posses more energy than cold water. Using the work energy theorem, you see that the change in kinetic energy is greater than the with the cold water, and will provide more work done on the hot water system than the cold.

I think its beacause a greater differnce in tempature allows a stronger system of heat transfer to be setup.

Like a tornadeo or hurricane being the symbol of strong trasfer. those weather systems that form when extreme frounts meet eachother allow for a better transfer.

reply to post by auraelium


its because boiling water creates steam and that is what starts to freeze first because there is more surface area in steam that come into contact with the cold therefor it freezes faster

reply to post by avocadoshag
[mor

Ice floats on cold water sure, however the change in density once in freezing cold air causes it to fall, once it makes contact with the water, it melts, cold water has a higher density than warm so it sinks, warmer water continues to rise-evaporate-freeze-sink-melt-sink until evaporation ceases by which time the water is very cold, colder than if the water had been 'stiller' and without the evaporation-freezing cycle. It causes a convection cycle, preventing the ice from forming at the top of the water and encouraging the cooling from evaporation.

It must certainly be at least part of the equation.

Originally posted by Infi8nity
Heat expands thus having more contact with the cold air?

BINGO.

My dad, who didn't graduate high school, told me this when I was kid every year when we used to toss boiling water in the yard to make a skating rink.

ALSO, it's super hot so since it's exposed to cold atmosphere it cools at a rapid rate from +100c to -0c in no time.

Real life test outside the lab

Originally posted by LogCog
reply to post by Bedlam

 

The fact is that both reasons are correct but there are differing opinions even today as to which factor in the cooling/freezing process is the most important. My guess is the currents and yours is gas dissolution. Let's get together and do some science!

If I wasn't heading off to the customer's site after three glorious weeks at home, I'd try to find a way to do this. I'll give it a few brain cells in my off moments. There ought to be a way to eliminate both/either factors from the experiment so only one thing is tested.

aliens ?

as others have stated its due to the movement. pretty simple really

I thought that it had something to do with the fact that in hot water, the molecules are already moving and there's no need to "jump start them" to get them into a different state, where as cold water would be a "standing start".

Where do I pick up my 1000 pounds?