water freezing

[sammmo]

I heard a very strange comment on the radio this morning..Someone was talking about feeding wild animals and birds during the bout of cold weather we are about to get and he said about putting out water for them, He said " Dont put out warm water because believe it or not, warm water freezes quicker and for longer than cold water" Well how does that work?

[JackDanielsU]

The effect is named for the Tanzanian citizen Erasto Mpemba. Mpemba first encountered the phenomenon in 1963 in Form 3 of Magamba Secondary School, Tanganyika when freezing hot ice cream mix in cookery classes and noticing that they froze before cold mixes. After passing his O-level examinations, he became a student at Mkwawa Secondary (formerly High) School, Iringa, Tanzania. The headmaster invited Dr. Denis G. Osborne from the University College in Dar Es Salaam to give a lecture on physics. After the lecture, Erasto Mpemba asked him the question "If you take two similar containers with equal volumes of water, one at 35 °C (95 °F) and the other at 100 °C (212 °F), and put them into a freezer, the one that started at 100 °C (212 °F) freezes first. Why?" only to be ridiculed by his classmates and teacher. After initial consternation, Dr. Osborne experimented on the issue back at his workplace and confirmed Erasto's finding. They published the results together in 1969.


Osborne observed that the top is warmer than the bottom in a beaker of water being cooled, the difference being sustained by convection. Blocking heat transfer from the top with a film of oil drastically slowed cooling. Also, the effect of dissolved air was accounted for by using boiled water. The beakers were also insulated from the bottom.

At first sight, the behaviour seems contrary to thermodynamics. Many standard physical theory effects contribute to the phenomenon, although no single explanation is conclusive. Several effects may contribute to the observation, depending on the experimental set-up:

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

[chazza]

I've heard this and we tested it, making ice cubes with both warm and cold water and putting them side by side in the freezer. The ones made with cold water froze much quicker than the warm water ones!

[Hopkirk]

I remember being told at school (a long time ago) about a strange property of water.

Apparently cold water is heavier than warm water down to 4 degrees c, then the reverse is true. That is why ice forms on the top of a pond, not at the bottom.

Apparently this fact is vital to life, as it means that fish will survive in water beneath ice. If cold water always dropped below warm water, then fish would have no chance.

Perhaps this effect has some bearing on the question here.

[NazNomad]

Interesting and very relevant ... I've linked to this in the Twitcher's thread, thanks.
http://www.theanswerb...uestion958202-11.html