When solid magnesium oxide (MgO) reacts with water (H2O), which hydroxide compound is formed as the main product of this combination reaction?

Introduction / Context:
This question is about a simple inorganic reaction between a basic oxide and water. Magnesium oxide is a metal oxide that shows basic character. When it reacts with water, it forms a metal hydroxide. Understanding this type of reaction helps students recognise general patterns in the chemistry of metal oxides and hydroxides, and it reinforces the idea that basic oxides often produce alkaline solutions when dissolved in water.

Given Data / Assumptions:

• Reactants: magnesium oxide (MgO) and water (H2O).
• The reaction is a combination type reaction between an oxide and water.
• We are looking for the correct empirical formula of the magnesium hydroxide product.
• Magnesium has valency 2 in its common compounds.

Concept / Approach:
Magnesium oxide is a basic oxide of a divalent metal. When a metal oxide like MgO dissolves or reacts with water, it forms the corresponding metal hydroxide. For magnesium, the common hydroxide is magnesium hydroxide with the formula Mg(OH)2. The balanced chemical equation is MgO + H2O gives Mg(OH)2. In this compound, magnesium has a charge of +2 and each hydroxide ion OH has a charge of -1, so two hydroxide ions are required to balance the charge on one magnesium ion.

Step-by-Step Solution:

Verification / Alternative check:
Another way to verify the formula is to check charge balance and atom balance. In Mg(OH)2, there is one magnesium atom, two oxygen atoms, and two hydrogen atoms. Combining MgO and H2O gives exactly that composition: one Mg, two O, and two H atoms. The ions Mg2+ and 2 OH- combine to give a neutral compound. Magnesium hydroxide is also known in everyday life as the active ingredient in some antacid medicines, and its formula is widely given as Mg(OH)2 in textbooks and product information. This independent confirmation agrees with the reasoning above.

Why Other Options Are Wrong:

• Mg(OH): This would imply a +1 charge on magnesium, which is incorrect for its usual chemistry and does not form a stable neutral compound.
• Mg2(OH): This formula is not charge balanced and does not match the stoichiometry of the reaction with water.
• Mg(O2H)2: This is not a standard way to write the hydroxide and misrepresents the arrangement of the atoms.
• Mg2(OH)2: This would suggest two magnesium ions and two hydroxide groups, which would not be neutrally balanced without additional counter ions.

Common Pitfalls:
Students sometimes forget to match the valency of the metal with the number of hydroxide groups and may try to guess the formula by trial. Another mistake is to misplace brackets when writing hydroxide formulas, leading to confusion about how many OH groups are present. A good habit is to first write the charges of the ions, cross multiply them to get the correct ratio, and then check the total number of each atom and the overall neutrality of the compound.

Final Answer:
When magnesium oxide reacts with water, the main product formed is Mg(OH)2, magnesium hydroxide.