why do we consider CuO as a base?

According to Arrhenius acid/base theory, a base is a substance which when dissolved in water furnishes $\ce{OH-}$.


But $\ce{CuO}$ is insoluble in water and does not produce $\ce{OH-}$, so why do we consider it a base?




There are two other widely used acid/base theories that apply here: [Brønsted–Lowry theory](http://en.wikipedia.org/wiki/Br%C3%B8nsted%E2%80%93Lowry_acid%E2%80%93base_theory) and [Lewis theory](http://en.wikipedia.org/wiki/Lewis_acids_and_bases).


A Brønsted base is defined as a proton acceptor. Technically, $\ce{CuO}$ is capable of reacting with $\ce{H+}$ to form hydroxide ions and eventually $\ce{H2O}$. Although $\ce{CuO}$ is insoluble in water (as you said), this reaction can occur in other solvents or in a biphasic system (where the proton is in solution and the $\ce{CuO}$ is present as an undissolved solid).


A Lewis base is defined as an electron donor. Generally, this means it has a lone pair of electrons that is available for reactions. The oxygen atom in $\ce{CuO}$ has lone pairs of electrons available.


$\ce{CuO}$ is considered a base because it fits these two descriptions.


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An additional way to consider it: $\ce{CuO}$ is defined as a base in the Lux-Flood theory because it is an oxide ion donor; it donates oxide to oxide acceptors ([Lux-Flood acids](http://en.wikipedia.org/wiki/Lux-Flood_theory))


An example of a Lux-Flood acid-base reaction involving $\ce{CuO}$ is the high pressure reaction with carbon dioxide to give $\ce{CuCO\_3}$ ([source](http://onlinelibrary.wiley.com/doi/10.1002/zaac.19744100207/abstract)).


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$\ce{CuO}$ is a basic oxide and when it reacts with water it gives $\ce{Cu(OH)2}$ which is a base as it gives $\ce{Cu^2+}$ and $\ce{OH-}$ ions when dissolved in water.
So technically $\ce{CuO}$ is a base rather a basic oxide.
Generally all metallic oxides and hydroxides are bases.


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I checked with an expert and they said Copper Oxide is considered to be "sparingly soluble". And is considered to be an Arrhenius base.


So I think that might solve the conundrum. It's not considered insoluble.


From what I can tell, Al2O3 (Aluminium Oxide), is considered to be insoluble, and it's said that it thus wouldn't form Hydroxide ions in water. So Al2O3 is not an Arrhenius base.


Another technicality is


<https://www.khanacademy.org/science/chemistry/acids-and-bases-topic/acids-and-bases/a/arrhenius-acids-and-bases> "Note that depending on your class—or textbook or teacher—non-hydroxide-containing bases may or may not be classified as Arrhenius bases."


Hence <https://freechemistryonline.com/arrhenius-concept-of-bases.html> "a base was defined as a compound containing OH groups which can become hydroxide ions when the base is dissolved in water." and "The Arrhenius definitions do not include such compounds as ammonia (NH3) and calcium oxide (CaO)" and "CaO is termed a basic oxide but not a base"


So on that definition of Arrhenius base, where it has to contain an OH group, Copper Oxide wouldn't be considered an Arrhenius base.


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