Question #8906b

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Answer 1 · 2015-06-28T11:21:00

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

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.