Crosswords5 mins ago
De-icer spray
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I know how salt lowers the freezing point of water to inhibit it freezing, but how on earth does a de-icer spray melt it?
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Alcohols dissolve because they have a hydroxyl (OH) group attached to the molecule. This chemical group is very polar (pardon the pun in the context of de-icer). The oxygen atom loves electrons and pulls them towards itself very strongly, including the electron from the hydrogen that is bonded to it.
This makes the hydrogen a little bit positive, and the oxygen a little bit negative and enables it to take part in a process called hydrogen bonding. This is what makes water 'sticky' and why you can bend a stream of water with the static electricity on a comb, because all of the molecules cling together by the positive hydrogens attracting the negative oxygens on adjacent molecules.
So the alcohol can dissolve in water because it can take part in the same hydrogen bonding process. But because the alcohol molecule is much bigger than a water molecule, and a funny shape, it prevents the water molecules lining up so easily to get close enough together to form a solid crystal i.e. freeze. To do that you need to make the conditions much much colder. This is how antifreeze works. A big ethylene glycol molecule links up with lots of water molecules (and dissolves quite happily), but because it is a funny shape it prevents big regular ice crystals from forming, so even if you car radiator does begin to freeze the best it can do is form slush which won't burst your pipes.
So how does the de-icer melt the ice to start with? Well, the alcohol in the tin is concentrated. When you add it to the ice on the window, the diluting effect of the ice and concentrated alcohol mixing produces a little bit of heat which speeds up the melting process. Then the alcohol and water mix thoroughly, the alcohol dissoves in the water, lowering its melting point and preventing re-freezing.
Occasionally you can make the ice re-freeze but this is usually on an exceptionally cold day with a particularly thick layer of ice, so
Alcohols dissolve because they have a hydroxyl (OH) group attached to the molecule. This chemical group is very polar (pardon the pun in the context of de-icer). The oxygen atom loves electrons and pulls them towards itself very strongly, including the electron from the hydrogen that is bonded to it.
This makes the hydrogen a little bit positive, and the oxygen a little bit negative and enables it to take part in a process called hydrogen bonding. This is what makes water 'sticky' and why you can bend a stream of water with the static electricity on a comb, because all of the molecules cling together by the positive hydrogens attracting the negative oxygens on adjacent molecules.
So the alcohol can dissolve in water because it can take part in the same hydrogen bonding process. But because the alcohol molecule is much bigger than a water molecule, and a funny shape, it prevents the water molecules lining up so easily to get close enough together to form a solid crystal i.e. freeze. To do that you need to make the conditions much much colder. This is how antifreeze works. A big ethylene glycol molecule links up with lots of water molecules (and dissolves quite happily), but because it is a funny shape it prevents big regular ice crystals from forming, so even if you car radiator does begin to freeze the best it can do is form slush which won't burst your pipes.
So how does the de-icer melt the ice to start with? Well, the alcohol in the tin is concentrated. When you add it to the ice on the window, the diluting effect of the ice and concentrated alcohol mixing produces a little bit of heat which speeds up the melting process. Then the alcohol and water mix thoroughly, the alcohol dissoves in the water, lowering its melting point and preventing re-freezing.
Occasionally you can make the ice re-freeze but this is usually on an exceptionally cold day with a particularly thick layer of ice, so
Ooops some missing, scroll down for full reply
http://www.thenakedscientists.com/forum/index. php?topic=350.msg13367
http://www.thenakedscientists.com/forum/index. php?topic=350.msg13367
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