A white powder is known to be either calcium oxide (CaO) or calcium sulfate (CaSO4). Which test would best distinguish between them?
Show the answer and explanation
Correct answer
B. Add water and test the pH of the resulting solution with universal indicator.
Principle or equation
Calcium oxide is a basic oxide that reacts with water to form calcium hydroxide, producing an alkaline solution (pH > 7). Calcium sulfate is sparingly soluble and does not produce a strongly alkaline solution.
Why this answer is correct
When water is added, CaO reacts exothermically to form Ca(OH)2, which is soluble enough to give a pH around 12-13. CaSO4 is only slightly soluble and the solution remains near neutral. Thus pH testing distinguishes them. Option A is incorrect because CaSO4 does not react with dilute acid to produce CO2. Option C is incorrect because neither compound decomposes to give O2 under simple heating. Option D is incorrect because sulfate gives a white precipitate with barium chloride, not silver nitrate (silver sulfate is slightly soluble but not a reliable test here).
Example
Add a small amount of water to each sample. If the powder is CaO, the mixture becomes hot and the solution turns universal indicator blue/purple. If it is CaSO4, little heat is evolved and the indicator stays green/yellow.
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