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Old December 7th, 2010, 04:18
fify fify is offline
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Default Why are alcohols liquids?

What is the reason that alcohols are not gases please?
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Old December 7th, 2010, 06:37
NanoMachine NanoMachine is offline
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This is usually asked in the context of something like "why are ethane, propane and butane gases, while ethanol, propanol and butanol are liquids". (We're talking about room temperature here. Heat them up and all these alcohols will vaporize to gases.)

The answer is that in comparing each alkane to its corresponding alcohol, you've replaced H with OH. OH introduces polarity into each molecule. The O has a slight overall negative charge on it while the H it's attached to has a slight positive charge. This means the O on one molecule of ethanol can now attract an H on an entirely different molecule. This attractive effect extends throughout all the molecules, pulling them together into the liquid phase. Alcohols are liquids, not gases because of inter-molecular polar bonding.
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Old December 7th, 2010, 08:18
fify fify is offline
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Thank you NanoMachine. This helps me a lot. Is water a liquid for the same reason alcohols are liquids?
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Old December 7th, 2010, 09:05
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Short answer - yes.

Longer answer - yes, and hydrogen bonding is even more significant for water than for for alcohols.

Water only consists of H - O - H and so every atom in water takes part in hydrogen bonding. Hydrogen bonds are very "concentrated" in water. In alcohols the hydrogen bonds are restricted to the OH part of the molecule and the C - C and C - H don't take part in hydrogen bonding, diluting the hydrogen bonding effect compared with water.

For such a light molecule, water's boiling point is high, 100oC.

The much heavier ethanol boils at just 78.1oC because, although hydrogen bonding is important in helping ethanol molecules stick together, it's not as important as it is in water.
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Old December 7th, 2010, 09:28
fify fify is offline
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Thank you again. So the statement is correct that hydrogen bonding is intermolecular bonding between oxygen on one molecule and hydrogen on another. The statement is also correct that water would be a gas at room temperature and there would be no life on earth without hydrogen bonding. Yes?
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Old December 7th, 2010, 10:31
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Both statements are right.

We also have hydrogen bonding to thank for ice floating on water. Very few solids are less dense than their liquids, but that's the way it is with water ice. If ice didn't float on water, it would sink to the bottom of the oceans, which would be bad news for life on Earth. (Ice does sink in alcohol.)

Hydrogen bonding is also important in DNA and proteins, so there are many reasons that life wouldn't exist without hydrogen bonding.
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