Incorrect order in sulfate ion test procedure Steps:

• Option A: Confirm MgSO4 solubility exceeds 30g/100mL water, allowing safe digestive use as a laxative.
• Option B: CaCO3 neutralizes H+ ions in acidic soil via CaCO3 + 2H+ → Ca2+ + H2O + CO2, raising pH from low values.
• Option C: Standard test acidifies solution first with HNO3, then adds Ba(NO3)2 to form insoluble BaSO4; reverse order risks temporary precipitates from CO3^2- or SO3^2-.
• Option D: Ba(OH)2, though toxic, lowers stomach acidity like other bases, but statement assumes safe use despite solubility issues.

Why C is correct:

• Test protocol (per qualitative analysis standards) requires pre-acidification to eliminate interfering anions; post-addition of HNO3 confirms but does not initiate properly.

Why the others are wrong:

• A: Statement matches MgSO4's high solubility and medical application.
• B: CaCO3 application corrects low soil pH by neutralization.
• D: Ba(OH)2 acts as base to reduce H+ in stomach, with "insoluble" referring to limited reactivity.

Final answer: C