So you fill your ice-cube tray from the hot tap and place it in the freezer. Panic over. In the latest twist, two physicists have mapped out a generalized theoretical framework for how such an unusual event might occur in simple systems. If true, it would be welcome news for those who believe cold water boils faster than warm or hot water, which has been largely dismissed to date as a scientific myth. Their work has also inspired scientists from Spain to devise their own theoretical model showing that the Mpemba effect could occur in granular fluid consisting of spheres suspended in a liquid.
The notion that hot water freezes faster than cold is named after Erasto Mpemba. In fact, not cooling their milk before freezing was common practice among local ice-cream vendors at the time.
Osborne promised to try the experiment when he returned to his university. Personally, he thought the boy was mistaken, but felt no question should be ridiculed, and conceded there might be other unknown factors affecting the rate of cooling.
The Mpemba effect has been a staple of DIY educational home experiments ever since, but he was not the first to notice it. Around BC Aristotle observed that it was local custom to put water in the Sun first if one wanted the liquid to cool more quickly. And over the last 10—15 years, scientists have been looking more closely at the Mpemba effect, hoping to tease out the precise causes of such a counter-intuitive phenomenon. Rival explanations One of the most common explanations put forward by scientists over the years centres on the influence of convective heat transfer, in which water forms convection currents as it heats, transferring hot liquid to the surface, where it evaporates.
As a result of this effect, an open cup with hot water would evaporate more quickly than a similar vessel with cool water, with the remaining liquid therefore freezing faster. But this would limit the effect to open-topped vessels, and some experiments have observed the effect in closed vessels too. Supercooling — where water can remain a liquid at well below its usual freezing point — may also be involved, provided that the water is sufficiently free of impurities, which otherwise help liquids crystallize into a solid.
His experiments revealed that the Mpemba effect occurs when ice crystals appear in a supercooled liquid at higher temperatures, which means that, in such cases, hot water would appear to freeze first. In , however, Jonathan Katz from Washington University in St Louis suggested that perhaps solutes like calcium carbonate or magnesium carbonate in cold water hold the key — they slow down the freezing process, giving hot water the edge Am.
More recently, chemists running molecular simulations have suggested the Mpemba effect might be linked to the unusual nature of hydrogen bonding in water J. Theory and Comp. These inter-molecular bonds, which are weaker than the covalent bonds holding the hydrogen and oxygen atoms within each molecule together, break up when water is heated. The water molecules then form fragments and realign into the crystalline structure of ice, kicking off the freezing process.
Since cold water must first break those weak hydrogen bonds before freezing can begin, it makes sense that hot water would start to freeze before cold.
Unfortunately, none of these proposed explanations have proven convincing enough to sway sceptical scientists. And more recent attempts to reproduce the effect consistently in lab experiments have been inconclusive.
Despite his best efforts at uniformity, some trays started freezing within 15 minutes, others took more than an hour. That kind of high variability is typical of Mpemba experiments. Some scientists doubt the effect even exists at all.
I'd wager the quart of water initially hot will come to a boil in much less time than the quart of water initially cold. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Learn more. Does cold water boil faster than hot water? Ask Question. Asked 4 years, 6 months ago. Active 4 years, 6 months ago. Viewed 38k times. Improve this question. Coomie Coomie 8, 11 11 gold badges 42 42 silver badges 78 78 bronze badges. I don't know if the inverse of notable claim is automatically notable as well. Anyway, here is an interesting and related study: Questioning the Mpemba effect: hot water does not cool more quickly than cold — Jordy.
Fill a kettle with cold water and time how long it takes to boil. Do the same except with the same amount of hot water. Fred W. Decker, a meteorologist at Oregon State University in Corvallis, encourages readers to settle the question for themselves:. Use a given setting on an electric hot plate and clock the time between start and boiling for a given pot containing, say, one quart of water; first start with the water as cold as the tap will provide and then repeat it with the hottest water available from that tap.
I'd wager the quart of water initially hot will come to a boil in much less time than the quart of water initially cold. Take into the chamber two quart-volume milk bottles filled with water, one from a hot tap and the other from a cold tap outside the chamber. Time them to freezing, and I would wager again that the initially colder water will freeze sooner than the initially hot water. Decker concludes that "much folklore results from trying to answer such a question under conditions that do not make 'all other things equal,' which the foregoing experiments do.
Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Decker, a meteorologist at Oregon State University in Corvallis, encourages readers to settle the question for themselves: "You can readily set up an experiment to learn which freezes earlier: water that is initially hot, or water that is initially cold.
0コメント