It has an empirical formula of Mg O and consists of a lattice of Mg 2+ ions and O 2− ions held together by ionic bonding. Magnesium hydroxide forms in the presence of water (MgO + H 2 O → Mg(OH) 2), but it can be reversed by heating it to separate moisture.agnesium oxide is easily made by burning magnesium ribbon, which produces a very bright white light, and a powdery ash. The bright flame is very hard to extinguish and it emits a harmful intensity of UV light. Inhalation of magnesium oxide fumes can cause metal fume fever.