Comme déjà indiqué ici, les scientifiques savent depuis Aristote que l'eau chaude gèle plus vite que l'eau tiède.
Une nouvelle explication de ce phénomène, appelé effet Mpemba, vient d'être proposée par une équipe de chercheurs de Singapour.
As everyone knows, water molecules
have one oxygen atom and two hydrogen atoms—all held together by
covalent bonds (the sharing of electrons). What's also known is that
with water molecules, hydrogen atoms are also attracted to the oxygen
atoms in other nearby water molecules—a force called a hydrogen bond.
But, at the same time, the water molecules as a whole are repelled by
one another. The team in Singapore has noted that the warmer water gets,
the more distance there is between the water molecules due to the
repellant force between them. That they say, forces the hydrogen bonds
to become stretched out, and stretching out a bond means that there is
energy being stored. That energy, the researchers suggest, is released
as the water is cooled allowing the molecules to become closer to one
another, and (as every chemistry student knows) giving up energy means
cooling.
Warm water has more hydrogen bond stretching going on than cool water, thus it stores more energy, and has more to release when exposed to freezing temperatures. That is why, the researchers say, it freezes faster than cool water.
At this point, the claims by the research team are still theory—they or others will still need to find a way to prove what they've suggested is true before the scientific community will deem the mystery of warm water freezing, solved once and for all.
Read more at: http://phys.org/news/2013-11-faster-cooler.html#jCp
Warm water has more hydrogen bond stretching going on than cool water, thus it stores more energy, and has more to release when exposed to freezing temperatures. That is why, the researchers say, it freezes faster than cool water.
At this point, the claims by the research team are still theory—they or others will still need to find a way to prove what they've suggested is true before the scientific community will deem the mystery of warm water freezing, solved once and for all.
Read more at: http://phys.org/news/2013-11-faster-cooler.html#jCp
As everyone knows, water molecules
have one oxygen atom and two hydrogen atoms—all held together by
covalent bonds (the sharing of electrons). What's also known is that
with water molecules, hydrogen atoms are also attracted to the oxygen
atoms in other nearby water molecules—a force called a hydrogen bond.
But, at the same time, the water molecules as a whole are repelled by
one another. The team in Singapore has noted that the warmer water gets,
the more distance there is between the water molecules due to the
repellant force between them. That they say, forces the hydrogen bonds
to become stretched out, and stretching out a bond means that there is
energy being stored. That energy, the researchers suggest, is released
as the water is cooled allowing the molecules to become closer to one
another, and (as every chemistry student knows) giving up energy means
cooling.
Warm water has more hydrogen bond stretching going on than cool water, thus it stores more energy, and has more to release when exposed to freezing temperatures. That is why, the researchers say, it freezes faster than cool water.
At this point, the claims by the research team are still theory—they or others will still need to find a way to prove what they've suggested is true before the scientific community will deem the mystery of warm water freezing, solved once and for all.
Read more at: http://phys.org/news/2013-11-faster-cooler.html#jCp
Warm water has more hydrogen bond stretching going on than cool water, thus it stores more energy, and has more to release when exposed to freezing temperatures. That is why, the researchers say, it freezes faster than cool water.
At this point, the claims by the research team are still theory—they or others will still need to find a way to prove what they've suggested is true before the scientific community will deem the mystery of warm water freezing, solved once and for all.
Read more at: http://phys.org/news/2013-11-faster-cooler.html#jCp
La molécule d'eau est constituée de deux atomes d'hydrogène et d'un atome d'oxygène, reliés entre eux par des liaisons covalentes (partages d'électrons), comme vous le savez sans doute.
Aussi, les atomes d'hydrogène sont attirés par les atomes d'oxygène des molécules d'eau voisines (liaison hydrogène). Dans le même temps, les molécules d'eau se repoussent les unes les autres.
Ce qu'expliquent les chercheurs asiatiques, c'est que plus l'eau se réchauffe, plus la distance entre les molécules augmente, ce qui étire les liaisons hydrogène.
Or, comme lorsque l'on tire un élastique, étirer les liaisons de la sorte revient à stocker de l'énergie.
C'est précisément cette énergie qui est relâchée quand l'eau se refroidie, les molécules se rapprochant alors les unes des autres.
Étant donné que l'eau chaude a des liaisons hydrogènes plus étirées que l'eau froide, elle stocke plus d'énergie et en relâche logiquement plus lorsqu'elle gèle, ce qui explique qu’elle change d'état plus rapidement...en théorie !
En effet, cela reste une hypothèse, qui vient s'ajouter à celles déjà formulées ici, pour expliquer ce phénomène décidément très mystérieux.
En savoir plus:
La publication scientifique (en anglais) est disponible ici: http://arxiv.org/pdf/1310.6514v1
As everyone knows, water molecules
have one oxygen atom and two hydrogen atoms—all held together by
covalent bonds (the sharing of electrons). What's also known is that
with water molecules, hydrogen atoms are also attracted to the oxygen
atoms in other nearby water molecules—a force called a hydrogen bond.
But, at the same time, the water molecules as a whole are repelled by
one another. The team in Singapore has noted that the warmer water gets,
the more distance there is between the water molecules due to the
repellant force between them. That they say, forces the hydrogen bonds
to become stretched out, and stretching out a bond means that there is
energy being stored. That energy, the researchers suggest, is released
as the water is cooled allowing the molecules to become closer to one
another, and (as every chemistry student knows) giving up energy means
cooling.
Warm water has more hydrogen bond stretching going on than cool water, thus it stores more energy, and has more to release when exposed to freezing temperatures. That is why, the researchers say, it freezes faster than cool water.
At this point, the claims by the research team are still theory—they or others will still need to find a way to prove what they've suggested is true before the scientific community will deem the mystery of warm water freezing, solved once and for all.
Read more at: http://phys.org/news/2013-11-faster-cooler.html#jCp
Warm water has more hydrogen bond stretching going on than cool water, thus it stores more energy, and has more to release when exposed to freezing temperatures. That is why, the researchers say, it freezes faster than cool water.
At this point, the claims by the research team are still theory—they or others will still need to find a way to prove what they've suggested is true before the scientific community will deem the mystery of warm water freezing, solved once and for all.
Read more at: http://phys.org/news/2013-11-faster-cooler.html#jCp
As everyone knows, water molecules
have one oxygen atom and two hydrogen atoms—all held together by
covalent bonds (the sharing of electrons). What's also known is that
with water molecules, hydrogen atoms are also attracted to the oxygen
atoms in other nearby water molecules—a force called a hydrogen bond.
But, at the same time, the water molecules as a whole are repelled by
one another. The team in Singapore has noted that the warmer water gets,
the more distance there is between the water molecules due to the
repellant force between them. That they say, forces the hydrogen bonds
to become stretched out, and stretching out a bond means that there is
energy being stored. That energy, the researchers suggest, is released
as the water is cooled allowing the molecules to become closer to one
another, and (as every chemistry student knows) giving up energy means
cooling.
Warm water has more hydrogen bond stretching going on than cool water, thus it stores more energy, and has more to release when exposed to freezing temperatures. That is why, the researchers say, it freezes faster than cool water.
At this point, the claims by the research team are still theory—they or others will still need to find a way to prove what they've suggested is true before the scientific community will deem the mystery of warm water freezing, solved once and for all.
Read more at: http://phys.org/news/2013-11-faster-cooler.html#jCp
Warm water has more hydrogen bond stretching going on than cool water, thus it stores more energy, and has more to release when exposed to freezing temperatures. That is why, the researchers say, it freezes faster than cool water.
At this point, the claims by the research team are still theory—they or others will still need to find a way to prove what they've suggested is true before the scientific community will deem the mystery of warm water freezing, solved once and for all.
Read more at: http://phys.org/news/2013-11-faster-cooler.html#jCp
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