Which of the following gases is not acidic?

Analyze the question: We need the gas that is not an acidic oxide, meaning it does not react with water or bases to form an acid or a salt.

Concept: Acidic oxides are usually oxides of non-metals. They dissolve in water to form acids, or react with bases to form salts and water. Some non-metal oxides are neutral, and they do not do either. The common neutral oxides are CO, N₂O, NO and H₂O.

Check each option:

• A. SO₃: SO₃ + H₂O → H₂SO₄ (sulphuric acid). It is acidic.
• B. NO₂: 2NO₂ + H₂O → HNO₃ + HNO₂ (and with oxygen it gives HNO₃). It is acidic.
• C. CO₂: CO₂ + H₂O ⇌ H₂CO₃ (carbonic acid). It is acidic. (The option is written "Co₂", but it means carbon dioxide, CO₂. "Co" is the symbol for cobalt, and "CO₂" is the intended formula.)
• D. CO: Carbon monoxide does not react with water to form an acid, and it does not react with bases under ordinary conditions. It is a neutral oxide.

Answer: The gas that is not acidic is CO, which is option D.

Tip: Remember the neutral oxides as "CO, NO, N₂O, H₂O". Whenever a question asks for a non-acidic non-metal oxide, check this list first.