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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.

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solid oxide which dissolves in water

Magnesium oxide $\ce{MgO}$ is (practically) insoluble in water.

You might be interested to know that burning of magnesium was the classical flash light in the early days of photography: Wikipedia.

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    $\begingroup$ If you burn it under water (in the absence of oxygen) then it is of course reactive enough to form magnesium hydroxide (and hydrogen). $\endgroup$ – Martin - マーチン Sep 19 '14 at 16:41
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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.

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