A white powder is known to be either sodium oxide (Na2O) or sodium chloride (NaCl). Which test would best distinguish between them?
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Correct answer
A. Add water and test the pH of the resulting solution.
Principle or equation
Sodium oxide is a basic oxide that reacts with water to form sodium hydroxide, giving a strongly alkaline solution (pH > 7). Sodium chloride dissolves to give a neutral solution (pH ≈ 7).
Why this answer is correct
Step 1: Recognize that Na2O is a basic oxide. Step 2: When added to water, Na2O reacts: Na2O + H2O → 2NaOH, producing OH⁻ ions, so the solution is basic. Step 3: NaCl dissolves without hydrolysis, giving a neutral solution. Step 4: Testing pH distinguishes them. Adding dilute nitric acid: both dissolve, no effervescence for either (Na2O reacts but no gas). Flame color: both give yellow flame, not distinguishing. Silver nitrate: only NaCl gives white AgCl precipitate, but Na2O would also give Ag2O? Actually Ag2O is brown, but this test is less direct and not the best.
Example
If 0.5 g of the powder is added to 10 mL water, a pH paper shows pH 13 for Na2O and pH 7 for NaCl.
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