Blowing carbon dioxide gas into a solution of magnesium oxide will form magnesium carbonate (MgCO₃).

Here's the chemical equation:

MgO(s) + CO₂(g) → MgCO₃(s)

This reaction is a simple acid-base reaction. Magnesium oxide (MgO) is a basic oxide, while carbon dioxide (CO₂) dissolved in water forms carbonic acid (H₂CO₃), a weak acid. The reaction results in the formation of magnesium carbonate, a white solid.