Question #8906b

Jun 28, 2015

Simply put, because one is soluble and the other is not.

Explanation:

An alkali is a substance that dissolves in water to give a basic solution, i.e. a solution with a pH greater than 7.

In your case, potassium oxide, ${K}_{2} O$, will react with water to produce potassium hydroxide*, $K O H$, which is a strong base.

${K}_{2} {O}_{\left(s\right)} + {H}_{2} {O}_{\left(l\right)} \to 2 K O {H}_{\left(a q\right)}$

Sodium hydroxide will then dissociate completely in aqueous solution to form potassium cations, ${K}^{+}$, and hydroxide anions, $O {H}^{-}$

$K O {H}_{\left(a q\right)} \to {K}_{\left(a q\right)}^{+} + O {H}_{\left(a q\right)}^{-}$

So, dissolving potassium oxide in water will increase the hydroxide concentration, making the solution more basic $\to$ ${K}_{2} O$ is an alkali.

On the other hand, copper oxide is considered to be insoluble in water.

This means that the pH of the resulting solution will not change $\to$ $C u O$ is not an alkali.