# Which of the following elements forms a water-soluble, alkaline oxide?

An element burns in oxygen to form a solid oxide which dissolves in water. The pH of the resulting solution is greater than seven. Which one of the following could the element be?

A. sulphur

B. phosphorus

C. magnesium

D. sodium

E. carbon

The right answer is sodium. But I don't really understand why it can't be magnesium as well.

Magnesium oxide $\ce{MgO}$ is (practically) insoluble in water.
The answer is sodium. In fact, burning sodium in oxygen atmosphere will produce sodium oxide. When water is added to sodium oxide $\ce{NaOH}$ is produced. The $pH$ of the resulting solution is greater than seven. It can not be $\ce{Mg}$, because $\ce{MgO}$ is insoluble in water.