Which freezes faster - cold water or boiling water? Hailey, Sprighill, Florida
Usually cold water freezes before hot, as Newton theorized in 1701. But, hot water - even boiling water - can freeze faster than cold.
You Tube shows a video of university student Jim Sage on top of Mount Washington in heavy winter clothes boiling water on a small alcohol stove in the -34.8 F (-37.1 C) cold. Jim lifts the pot, and tosses boiling-hot water up then watches it snow down, like a blizzard, all around him.
That's fast freezing, but faster than cold water?
It can happen, as Erasto B. Mpemba discovered in 1967 when he was a form-3 student (equivalent to North American 11th grade) making ice cream at Magamba Secondary School in Lushoto, a small town in Tanzania, Africa.
Erasto asked his science teacher to explain why his hot milk froze faster than a friend's cold milk. The teacher said, "You were confused; that cannot happen."
Erasto must have had some reservations about this reply because, over the next couple of years, he asked the same question of two more physics teachers, finally finding one with an open mind.
Erasto's day of discovery started like most school days. He woke up before dawn. Pulling on his school-uniform navy-blue shorts, he shivered in the cold. But the afternoon would be hot, this close to the equator, even though he was almost a mile high in the mountains. "I've time to make ice cream," he thought, and set out for milk.
He strode briskly along the dirt main street of Lushoto to the open-air market. Quickly he bought some milk, and headed back to school. He mixed sugar with milk, and put the pot on the stove to boil. It had begun to boil, when he noticed another boy not bothering to heat his mixture in order to seize space in the refrigerator's freezing compartment. Erasto grabbed the last available space, putting his boiling-hot liquid beside the cool one.
"The other boy and I went back in an hour and a half later, and found that my tray of milk had frozen into ice-cream while his was only a thick liquid, not yet frozen," Erasto wrote later. Erasto asked his physics professor why, and got only 'that cannot happen' answer.
Two years later, in Mkwawa High School, Erasto again asked why the hot liquid froze faster than the cold. His new teacher answered: You were confused.
But help was coming. The school's headmaster invited physicist Dennis Osborne of the University College in Dar es Salaam to lecture and answer questions. Erasto asked his question yet again.
Dr. Osborne smiled into Erasto's earnest face. "Is it true, have you done it?"
Everyday events are seldom as simple as they seem, mused Osborne to himself. "I do not know, but I promise to try this experiment."
And he did, eventually getting similar results. Osborne found cold water near 20 degrees C (70 F) began to freeze in about 100 minutes; whereas hot water about 80 degrees C (175 F) began to freeze much quicker - in 40 minutes.
Water, a commonplace substance is anything but simple. Certainly, as Mpemba and Osborne discovered, its freezing behaviour is not characterized by its average temperature.
No doubt, several mechanisms underlie the Mpemba effect. We have eliminated some.
For example, a vessel containing initially hot water would melt frost on the floor of a non-frost-free refrigerator. Then, the hot-water vessel would be in direct contact with the cooling element of the freezer, and therefore freeze faster than the vessel with the initially cold water, which would rest on frost. The frost would insulate the cold-water vessel.
But, this mechanism doesn't explain the entire effect, because, in one experiment, Osborne placed both vessels on good thermal insulators. The hot water still froze faster than the cold.
Osborn thought perhaps evaporation could account for the hot water freezing faster, since the more active hot-water molecules would escape the liquid, cooling the remaining liquid. But he found only small changes in volume due to evaporation, and concluded evaporation could account for no more than 30 per cent of the cooling.
Three mechanisms remain that could explain the effect.
Hot-top. Cooling occurs mainly from the top surface. But hot water freezes differently than cold - more heat escapes from the top for hot than cold water.
Cold water starts to freeze along its top surface, and along its sides. The top ice insulates the water from the colder air above, slowing freezing. The insulated upper water does not chill appreciably more and, therefore, does not become denser and drop to the bottom. Few convection currents form, so little convective heat transfer occurs.