$\ce{CH3CH2CH2CHO}$
I think it is an aldehyde but I saw somewhere that it might be an alcohol...
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4$\begingroup$ that's an aldehyde $\endgroup$ – ron Feb 14 '17 at 19:29
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1$\begingroup$ In an alcohol, the alcohol group would be indicated as OH in a properly written formula. $\endgroup$ – bpedit Feb 14 '17 at 19:54
That's an aldehyde as it contains ($\ce{-CHO}$) group.
Its not an alcohol because in alcohol the carbon and oxygen bond in a single (sigma) bond whereas in aldehyde it is a double bond.
$\ce{CH3CH2CH2CHO}$ cant be a alcohol because as you can see that the terminal carbon is attached to only 3 groups ($\ce{C}$, $\ce{H}$ and $\ce{O}$) which means that it is still missing 1 bond to complete its octet. Therefore this carbon makes one more bond, a double bond, with the oxygen atom to complete its octet and the oxygen also attains a complete octet by this.