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I am confused on what is a hydroxide exactly is. For example, ethanol has a hydroxide group so does this mean it contains hydroxide ion? Does this make it ionic? I know it is a covalent molecule but why isn't it ionic?

Calcium hydroxide has $\ce{OH-}$ but how is this different than the $\ce{OH}$ group in ethanol? Is it covalent due to the fact it does not dissociate into ions when in aqueous solution? I know I'm missing something here.

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  • $\begingroup$ You must compare ethanol with water and sodium hydroxide with sodium ethoxide $\endgroup$ – Raoul Kessels Jun 6 '18 at 8:18
  • $\begingroup$ @RaoulKessels what do you mean? $\endgroup$ – user64524 Jun 6 '18 at 8:19
  • $\begingroup$ Sodium or calcium hydroxide has $\ce{OH^-}$ and sodium or calcium ethoxide has $\ce{EtO^-}$, while water is $\ce{H-OH}$ and ethanol $\ce{Et-OH}$. You can not cross compare them $\endgroup$ – Raoul Kessels Jun 6 '18 at 8:24
  • $\begingroup$ @RaoulKessels So more or less one has ions and the other does not? So The OH in calcium hydroxide is already an ion where as in water it is not? Kinda confused with that. $\endgroup$ – user64524 Jun 6 '18 at 8:32
  • $\begingroup$ Yes, and the same with ethanol $\endgroup$ – Raoul Kessels Jun 6 '18 at 8:34
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No, it is not ionic.

Your doubts are somehow legitimate if you are not aware of the concept of electronegativity. As a rule of thumb if two atoms have similar electronegativity is harder to have ionic bonds. You can use Pauling scale of electronegativity to have some insights on it.

A difference of electronegativity of over 1.7 is likely to be ionic, and a difference of less than 1.7 is likely to be covalent.

Source: Atkins, Peter; Loretta Jones (1997). Chemistry: Molecules, Matter and Change. New York: W. H. Freeman & Co. pp. 294–295. ISBN 0-7167-3107-X.

This means you will often deal with ionic compounds made of one ion with a strong electronegativity and one with a weak electronegativity. In fact, ions are composed of anions and cations. With a lot of approximations, we can calculate the electronegativity of a group averaging the electronegativities of the components for $\ce{-OH}~\dfrac{3.44 + 2.82}2 = 3.13$.

In your example, $\ce{C2H5OH}$ ethanol the bond is with $\ce{C}$ ( electronegativity 2.55) so the difference is 0.58 hence covalent. If the bond is with $\ce{Ca}$ (1.00) the difference would be 2.12 which is greater than 1.7 hence is categorized as ionic.

Of course is not always easy to determine if it's ionic or not so in complex cases we can say that it has an ionic character o covalent character.

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