Question

Question #8906b

Answer 1

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_2O`, will react with water to produce potassium hydroxide*, `KOH`, which is a strong base.

`K_2O_((s)) + H_2O_((l)) -> 2KOH_((aq))`

Sodium hydroxide will then dissociate completely in aqueous solution to form potassium cations, `K^(+)`, and hydroxide anions, `OH^(-)`

`KOH_((aq)) -> K_((aq))^(+) + OH_((aq))^(-)`

So, dissolving potassium oxide in water will increase the hydroxide concentration, making the solution more basic `->` `K_2O` 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 `->` `CuO` is not an alkali.