Which of the following substances, when added to water, will produce a solution with a pH less than 7?
Show the answer and explanation
Correct answer
B. Sulfur dioxide
Principle or equation
Non-metal oxides that are acidic dissolve in water to form acids, lowering pH. Metal oxides are basic and form alkaline solutions.
Why this answer is correct
Sulfur dioxide (SO2) reacts with water to form sulfurous acid (H2SO3), an acidic solution with pH < 7. Sodium oxide (Na2O), calcium oxide (CaO), and potassium oxide (K2O) are basic oxides; they react with water to form strong bases (NaOH, Ca(OH)2, KOH), giving pH > 7.
Example
CO2 dissolves in water to form carbonic acid, making the solution acidic (pH < 7).
This published item includes a stored explanation and passed the platform’s publication workflow. It is independent preparation material, not a claim of an official or recalled examination question.