freezing water

[silverdale5]

why does hot water freeze quicker than cold ?????

[shammydodger]

http://math.ucr.edu/home/baez/physics/General/hot_water.html

It's called The Mpemba Effect and holds for certain experimental conditions.

[bernardo]

It doesn't. It's an urban myth. I know, because I tried it once. I put two half-mugfuls (identical mugs) of water into the deep freeze at the same time. The cold one started freezing well before the hot one.

[fo3nix]

bernardo: it's not an urban myth! though sometimes it can hard to get it to work. it's not totally understood. I suggest you read shammydodge's link.

http://en.wikipedia.org/wiki/Mpemba_effect

[silverdale5]

Hmmmmmmmmm. its a curious one this. apparently ut does happen under certain conditions, but not others. scientists are not totally sure why.......if they dont know we havent got much chance have we ?

[bernardo]

Oh!!! Weird. It seems that the correct answer is therefore "nobody knows".

[Mr Songman]

I was told many years ago....(many many years ago...when I was a mere pup)....that it is BOILED water not just hot water that freezes quicker because of the oxygen that has been displaced by the boiling.

This is how you get nice clear ice cubes instead of frosty opaque ones!!

Least....that's how I remember it!!

[fo3nix]

bernardo: yea, it's still a mystery.

blue peter time: get two 500ml bottles and fill both, pretty full, with regular cold tap water. put them in the freezer together, and leave over night (for about 7 hours). when you next check them, sometimes one of frozen and the other isn't. if it's like that, carefully take out the one that isn't, and pour it down the drain. as it comes out of the bottle it'll freeze and turn to ice in your sink! (it's a similar type of thing; again sometimes hard to make it work, and not properly understood).

[pawwaw17]

water under pressure has a lower freezing point than that at normal air pressure. so if you put a bottle in the freezer and left it the pressure might increse if freezing started to occour and if the bottle contracted a little. then as the water was poured into the sink the water could be bellow freezing and the pressure was normal the water could then freeze in the sink.

[fo3nix]

thats true :)

there are other, similar effects too though. not entirely cut-and-dry yet.