In a laboratory demonstration, a student adds hydrochloric acid to sodium hydrogencarbonate and collects the gas. How can the student confirm it is carbon dioxide?

1. A

   It supports combustion.

2. B

   It forms a blue precipitate.

3. C

   It extinguishes a burning splint.

4. D

   It turns limewater milky.

Appeared in: RRB Nsg. Superintendent-2026 (Shift -2nd)

Explanation

• The standard and specific confirmatory test for carbon dioxide (CO₂) is bubbling the gas through limewater (calcium hydroxide solution).
• CO₂ reacts with calcium hydroxide to form calcium carbonate, a white insoluble precipitate, which causes the solution to turn milky or cloudy.
• This reaction is unique to carbon dioxide under normal laboratory conditions, making it a reliable test.

Why Other Options Were Wrong

• Option A: This statement describes a property of oxygen, not carbon dioxide. CO₂ is non-flammable and does not support combustion.
• Option B: Carbon dioxide does not form a blue precipitate. This observation is typically linked to the presence of certain metal ions, like copper(II).
• Option C: Although CO₂ does extinguish a burning splint, this test is not confirmatory because it is not specific. Other non-combustible gases, like nitrogen or argon, also extinguish flames by displacing oxygen.

Related Visual

• Visual 1: Diagram: An illustration showing the experimental setup. Gas produced from a flask containing HCl and NaHCO₃ is passed through a delivery tube into a test tube containing clear limewater, showing the limewater turning milky.

Clinical Relevance

• Nursing practice connection: Knowing Confirmatory test for carbon dioxide gas helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
• While this is a basic chemistry question, understanding CO₂ is vital in nursing. The body produces CO₂ as a waste product of metabolism, and its levels in the blood are critical for maintaining acid-base balance.
• Capnography is a non-invasive monitoring technique that measures the concentration of CO₂ in expired air, providing crucial information about a patient's ventilation, perfusion, and metabolic status.
• The bicarbonate buffer system (H₂CO₃ ⇌ H⁺ + HCO₃⁻) is the most important buffer system in the blood, regulating pH. This system is directly linked to the amount of dissolved CO₂.

How to Approach the Question

• First, identify the core of the question: it asks for a confirmatory test, which means a specific and unique test for carbon dioxide.
• Recall or deduce the chemical reaction: Acid (HCl) + Bicarbonate (NaHCO₃) → Salt + Water + CO₂ gas.
• Evaluate each option's specificity for identifying CO₂.
• Option A (supports combustion) is the test for oxygen. Eliminate it.
• Option B (blue precipitate) is not a characteristic test for any common gas and is irrelevant to CO₂. Eliminate it.
• Consider Option C (extinguishes a burning splint). While true for CO₂, ask if any other gas does this. Yes, any inert or non-flammable gas will extinguish a flame. Therefore, it is not a confirmatory test. Eliminate it.

Concept Tested & Keywords

• Concept Tested: Confirmatory test for carbon dioxide gas
• Stem keywords: hydrochloric acid, sodium hydrogencarbonate, carbon dioxide, confirm
• Lead-in keywords: How can the student confirm
• Negative lead-in flag: false

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QJkds_OclGGP6PNzDh1Ero

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