Which of the following oxides forms a strongly basic solution when dissolved in water? A) SO₃, B) CO₂, C) MgO, D) CO, or E) P₄O₁₀.
Oxides are materials that contain oxygen in the oxidation state of minus two. And strongly basic solutions contain high concentrations of the hydroxide iron. And we’ll have high pH values, probably above pH 10. There’s a simple rule of thumb that we can fall back on here, pertaining to whether the oxide is derived from a metal or a nonmetal. An oxide derived from a metal like sodium will form basic solutions when it dissolves, while oxides derived from nonmetals will likely form acidic solutions. So when metal oxides dissolve, we expect the pH to be above seven. And when nonmetal oxides dissolve, we expect the pH to be below seven.
Now, let’s have a look at our candidates. SO₃ is the formula for sulphur trioxide. CO₂ is the formula for carbon dioxide. MgO is the formula for magnesium oxide, while CO is the formula for the other common oxide of carbon, carbon monoxide. And P₄O₁₀ is the formula for phosphorus pentoxide. The name derives from the empirical formula of this compound P₂O₅. To find the oxide that’s gonna form a basic solution, what we can do is find the position of all the elements that are not oxygen in each of these candidates. Roughly speaking, we can divide the periodic table into two parts. Everything to the right of the pink line is a nonmetal. Everything else we call a metal. So far, our first element is in group 16 and in period three. It’s very clearly a nonmetal. So sulphur trioxide we expect to form acidic solutions.
Carbon is in group 14 and period two of the periodic table. It’s very clearly a nonmetal. Therefore, we expect its oxides, carbon dioxide and carbon monoxide, to form acidic solutions. Magnesium, on the other hand, is all the way to the left in group two, very clearly a metal. Therefore, we expect magnesium oxide to form basic solutions. Lastly, phosphorus sits in group 15 and period three to the right of the pink line. It’s a nonmetal. So phosphorus pentoxide should form acidic solutions.
So it looks like we found our answer, but let’s make sure by looking at the reaction between magnesium oxide and water. One unit of magnesium oxide reacts with a water molecule to produce one unit of magnesium hydroxide. Magnesium hydroxide is highly soluble and dissolves to form magnesium two plus ions and hydroxide ions. So we have those readily available hydroxide ions we need to produce a strongly basic solution.
If you’re interested, here’s what happens with sulphur trioxide. One molecule of the SO₃ reacts with a water molecule to produce sulfuric acid. Carbon dioxide is less soluble and less reactive, but it does react with water to produce carbonic acid. Carbon monoxide is quite different. Carbon monoxide is about 50 times less soluble than carbon dioxide and reacts extremely slowly with water, even under extreme conditions to form formic acid. Under normal conditions though, solutions of carbon monoxide would not be particularly acidic. However, they would definitely not be basic. Lastly, phosphorus pentoxide reacts with water vigorously producing phosphoric acid, meaning we can be confident that the only oxide out of the five that forms a strongly basic solution when dissolved in water is magnesium oxide, MgO.