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Which substance is the most soluble in water?

  1. $\ce{CH3OH}$
  2. $\ce{CH4}$
  3. $\ce{C2H6}$
  4. $\ce{C2H5OH}$

I wasn't able to decide between 1 and 4, as both have hydroxides, but the answer is 1. How?

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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 H2O molecule acts as bipolar and the +ve pole forms some weak hydrogen attraction(name this 'bond' if you like) with the oxygen of alcohol. Now its obvious , the more bulkier the group around oxygen ,the less space around oxygen to form this bond. Hence CH3OH is more soluble in water than C2H5OH. After all , C2H5- group is more soluble than CH3- group.

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    $\begingroup$ Except that, as many others have stated, both methanol and ethanol are infinitely miscible in water at room temperature. $\endgroup$ – bon Dec 2 '15 at 23:48
  • $\begingroup$ 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. $\endgroup$ – Qwerty Dec 3 '15 at 15:21

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