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Why hot Water freeze faster than cold water??? |
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Answer» Bhai kaha se copy kr dalaKade sadda maththa bhi use kr liya krOk thanks The Mpemba effect is the observation that warm water freezes more quickly than cold water. The effect has been measured on many occasions with many explanations put forward. One idea is that warm containers make better thermal contact with a refrigerator and so conduct heat more efficiently. Hence the faster freezing. Another is that warm water evaporates rapidly and since this is an endothermic process, it cools the water making it freeze more quickly.None of these explanations are entirely convincing, which is why the true explanation is still up for grabs.Today Xi Zhang at the Nanyang Technological University in Singapore and a few pals provide one. These guys say that the Mpemba paradox is the result of the unique properties of the different bonds that hold water together.What’s so odd about the bonds in water? A single water molecule consists of a relatively large oxygen atom joined to two smaller hydrogen atoms by standard covalent bonds.But put water molecules together and hydrogen bonds also begin to play an important role. These occur when a hydrogen in one molecule comes close the oxygen in another and bonds to it.Hydrogen bonds are weaker than covalent bonds but stronger than the van der Waals forces that geckos use to climb walls.Chemists have long known that they are important. For example, water’s boiling point is much higher than other liquids of similar molecules because hydrogen bonds hold it together.But in recent years, chemists have become increasingly aware of more subtle roles that hydrogen bonds can play. For example, water molecules inside narrow capillaries form into chains held together by hydrogen bonds. This plays an important role in trees and plants where water evaporation across a leaf membrane effectively pulls a chain of water molecules up from the roots.Now Xi and co say hydrogen bonds also explain the Mpemba effect. Their key idea is that hydrogen bonds bring water molecules into close contact and when this happens the natural repulsion between the molecules causes the covalent O-H bonds to stretch and store energy. Then there is the strange Mpemba effect, named after a Tanzanian student who discovered that a hot ice cream mix freezes faster than a cold mix in cookery classes in the early 1960s. ... Another is that warm water evaporates rapidly and since this is an endothermic process, it cools the water making it freeze |
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