why does hot water freeze faster than cold water explain
Does hot water freeze faster than cold water? It seems obvious that the answer should be no, because all things being equal, hot water takes longer to cool down than cold water, and so it couldn't possible freeze faster. But observations over thousands of years, as well as countless modern experiments, have shown that the opposite is true - under carefully controlled conditions, hot water at times seems to freeze faster than cold water. How is this possible? Well, that's something that still has scientists baffled - in fact, they're still struggling to prove the effect exists in the first place, as the
of Derek Muller's new science channel, explains. It turns out that freezing water is a lot more complicated than you might think. As the video above explains, the phenomenon of hot water freezing faster than cold water is known as the, named after, a Tanzanian student who in 1963 was making ice cream as part of a school project. The students were meant to boil a mixture of cream and sugar, let it cool down, and then put it in the freezer. Worried about getting a spot in the freezer, Mpemba instead put his mixture in while it was still scorching hot. But after 1. 5 hours, his mixture had frozen, while his classmates' mixtures had not. Intrigued by this phenomenon, he went on to work with physics professor Denis Osborne, and together they were able to replicate the findings and showing that warm water freezes faster than cold water. It was the first peer-reviewed study on the effect, but as, observations date all the way to Aristotle in the 4th century BCE, who observed that hot water cooled sooner than cold during his experiments. В Sir Frances Bacon and Descartes also noted the phenomenon in their studies.
But what's the physics behind this strange phenomenon? As the, there are five proposed mechanisms for what's going on here: Frost melting: В Frost is an insulator, and so frosty cold water might keep its heat better than a warm beaker that melts the frost off its sides. Dissolved gasses:В There are more dissolved gasses in cold water than warm water, and researchers have predicted that this could play a role in cooling rates, although it's not clear how. В Supercooling:В We all know that water freezes at zero degrees Celsius, but sometimes it gets a lot colder before it freezes - a phenomenon known as supercooling. This occurs because ice needs a nucleation site, such as an air bubble or impurity in the water in order to form. So maybe warm water experiences less supercooling than cold water. Evaporation: В The hot water beaker loses more water molecules through evaporation, so there's less of it to freeze. Convection: В Finally, there's the idea that warm water might cool faster due to increased convection currents. These currents occur because water cools primarily from its surface and the sides of the beaker, causing cold water to sink and warm water to rise up and take its place. The currents are greater in warm beakers, and could affect cooling rates. В There's merit in all those ideas, but the problem is that experiments over the years have controlled for all these effects, and the results have been frustratingly inconsistent. Some labs have failed to show the Mpemba effect happening at all, while others show it happening even under varying conditions. So what's the answer? Well a suggests that maybe the Mpemba effect is being caused by something else entirely - and it has nothing to do with how quickly hot water cools.
We'll let that one to you in the video above, and don't forget to to his new channel. It's pretty crazy to think that after millennia of observations, we still have so much to learn about something as basic as freezing water. Science is the best. В Lots of you in the Northern Hemisphere will be in the middle of yet another winter, and some might even be experiencing the sub-zero temperatures to do some cool experiments, such as creating,В , or even tossing boiling water into air to create snow (although that last one requires caution, seriously). If you remember the dreadfully cold winter of 2013, you might have seen videos like the one we've posted below circulating as 'proof' of the crazy low temperatures. В But it turns out this demonstration is actually not that surprising - hot water is in fact known to freeze faster than cold water. There are records of this from as far back as Aristotle's time. In modern times, this counterintuitive property has been named the Mpemba effect, after a Tanzanian secondary school student who re-discovered this phenomenon back in 1963. Erasto Mpemba and other kids at his school often made ice cream using the school freezer - they would do this by boiling milk and mixing it with sugar, which then had to be cooled and placed in the freezer. One day, Mpemba rushed the process and stuck the milk in while still hot, and to his surprise, the ice cream formed quicker than for his classmate. Unsurprisingly, none of his teachers believed the 13-year-old. Mpemba later teamed up with a physics professor who visited his high school, and in 1969, which has since been replicated many times - most often with a similar result.
Although if you try this at home and fail, it's probably because the Mpemba effect is not a reliable phenomenon that happens every single time; in fact, there seem to be So what are they? Well, it's one of those somewhat unsatisfying cases where just because we know something happens, doesn't mean we entirely understand why. But the process of scientific speculation is still interesting, so here are some versions. The most commonly proposed hypothesis - and one that's probably somewhat responsible for the effect - is that hot water evaporates more quickly, losing mass and therefore needing to lose less heat in order to freeze. However, scientists have also demonstrated the Mpemba effect with closed containers where evaporation doesn't take place. Another theoretical speculation is that water develops convection currents and temperature gradients as it cools - a rapidly cooling glass of hot water will have greater temperature differences throughout, and lose heat more quickly from the surface, whereas a uniformly cool glass of water has less of a temperature difference, and there's less convection to accelerate the process. But this idea has not been entirely verified either. Other theories have been put forward, including or the effect that dissolved gasses in the water would have on the freezing process. Itвs likely that several of these actually come into play. In late 2013, a team of researchers from Singapore that the Mpemba paradox stems from the unique properties of chemical bonds in the water. A standard water molecule contains one oxygen atom and two hydrogen atoms joined to it by covalent bonds sharing electron pairs between the atoms.
But when you put several water molecules together, the hydrogen atoms will also form bonds with oxygen atoms in other molecules. These hydrogen bonds are what gives water some of its properties, such as having a relatively high boiling point, and becoming less dense when frozen. According to the researchers, when water boils, the molecules spread out, lengthening the hydrogen bonds - but because volume is limited, meanwhile the covalent bonds within individual molecules get compressed, storing away energy. If water is frozen at this state, the bonds release energy as an uncoiled spring, cooling down much more quickly than if the covalent bonds were less compressed. However, even though many headlines heralded this as the definitive explanation for the Mpemba effect, the idea is yet to be published in a peer-reviewed journal. , the theory is convincing, but lacks predictive power: "Xi and co need to use their theory to predict a new property of water that conventional thinking about water does not. For example, the shortened covalent bonds might give rise to some measurable property of the water that would not otherwise be present. The discovery and measurement of this property would be the coup de grГceВ that their theory needs. " Wired, this explanation is also not strongly supported in the scientific community. "When we talked to chemist Richard Zare of Stanford University, he was unconvinced of its merits and said he thinks the dominant force at play is evaporation. " Still, itвs fun to (carefully) try this at home.
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