Which substance is the most soluble in water?
- $\ce{CH3OH}$
- $\ce{CH4}$
- $\ce{C2H6}$
- $\ce{C2H5OH}$
I wasn't able to decide between 1 and 4, as both have hydroxides, but the answer is 1. How?
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asked Dec 2, 2015 at 4:55
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$\begingroup$ The question should have specified under what range of conditions. Ethanol and methanol are both completely miscible with water under normal lab conditions. Not so under extreme conditions. $\endgroup$– matt_blackSep 15, 2020 at 14:47
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$\begingroup$ The distinction between methanol and ethonol, otherwise miscible with water in any ratio, can be done by easiness of salting them out from the water phase. By addition of the enough salt to the alkohol solution, ethanol phase separates easier.. $\endgroup$– PoutnikDec 26, 2020 at 17:49
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3 Answers
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For primary alcohols, the trend is the longer the chain, the less soluble.
However, at room temperature, methanol, ethanol and propanol are all completely miscible with water.
At lower temperature, below the freezing point of water, there is finite solubility.
See the phase diagram that is Fig. 1 in CRYSTAL GROWTH OF WATER ETHANOL MIXTURE and that in Fig. 1 of Solid-Liquid Phase Diagram of the System Methanol-Water
Which alcohol would you choose for windshield deicing solution?
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Options 1 and 4 both have polar $\ce{C-O}$ bonds which make them soluble in water. The question is, which one is more soluble? To answer that, notice that the non polar part of methanol is smaller, so it will be more soluble.
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How does a alcohol get soluble in water? The $\ce{H2O}$ molecule acts as bipolar and the positive pole forms some weak hydrogen attraction (name this 'bond' if you like) with the oxygen of alcohol. Now it's obvious, the more bulkier the group around oxygen, the less space around oxygen to form this bond. Hence $\ce{CH3OH}$ is more soluble in water than $\ce{C2H5OH}$. After all, $\ce{-C2H5}$ group is more soluble than $\ce{-CH3}$ group.
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1$\begingroup$ Except that, as many others have stated, both methanol and ethanol are infinitely miscible in water at room temperature. $\endgroup$– bonDec 2, 2015 at 23:48
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$\begingroup$ Yes ,you are quite right. But I just wanted to say that if there is even a slight difference in their solubility( say at the 10th decimal place {solubility in mol/litre}), then its probable reason is the one I gave. $\endgroup$– QwertyDec 3, 2015 at 15:21