Mixing nonmetal oxides with water produces solutions that are A) acidic, B) basic, C) neutral, D) supersaturated, or E, unstable.
On the periodic table, elements are grouped according to similar properties, specifically the metals, nonmetals, and metalloid groupings. A metal oxide is a metal–oxygen compound. And examples of metal oxides are calcium oxide, zinc oxide, and sodium oxide. A nonmetal oxide is a nonmetal–oxygen compound. And examples include carbon dioxide, sulfur dioxide, and nitrogen dioxide.
When a metal oxide reacts with water, they produce hydroxides, which are known as bases. Using the examples given, we get the hydroxides calcium hydroxide, zinc hydroxide, and sodium hydroxide. These bases contain the hydroxide ion OH minus.
When nonmetal oxides react with water, we get acids. Using the examples given, the acids produced are H₂CO₃, or carbonic acid; H₂SO₃, sulfurous acid; and HNO₃, nitric acid. Acids all contain H plus ions. It is the number of H plus ions in solution which determines the pH and whether a solution is acidic or basic.
We know that the answer to our question is probably option A, acidic. But let’s just discuss pH a little bit further to confirm this.
We said that pH indicates the relative concentration of H plus ions in solution. H plus ions do not really exist by themselves but react with water to form hydronium ions. So pH is really a measure of the relative amount of hydronium ions in solution. A pH of seven is neutral, anything below seven is acidic, and anything above seven indicates a solution that is basic.
pH is actually an inverse relationship with the concentration of hydronium ions. So an acid having a low pH actually has a high concentration of hydronium ions. And a base with a high pH has a low concentration of hydronium ions. When a base dissolves in water, it reversibly dissociates into positive ions and hydroxide ions. When an acid dissolves in water, it reversibly dissociates into negative ions and H plus ions. The H plus ions react with water to form the hydronium ions, making the solution acidic.
In both acid and base solutions, water molecules react with themselves to form hydronium ions and hydroxide ions. So all acids and bases that are aqueous contain hydronium and hydroxide ions. That is why the pH scale, which uses hydronium ion concentration, can be used for both acids as well as bases.
The important thing to note is that, in an acid solution, the concentration of hydronium ions is high. So although acids and bases both contain hydronium ions, an acidic solution will be produced when a nonmetal oxide reacts with water. And so the answer is A, acidic.