Which of the following oxides is basic?

Step 1: Analyze the question We need to find which oxide is basic. The behavior of an oxide depends on the type of element it contains.

Step 2: The concept

• Metal oxides (especially Group 1 and 2) are usually basic. They react with water to form alkalis and with acids to form salts and water.
• Non-metal oxides are usually acidic. They react with water to form acids.
• Amphoteric oxides (such as Al₂O₃, ZnO and PbO) react with both acids and bases.

Step 3: Check each option

• A. NO₂: Nitrogen is a non-metal, so this is an acidic oxide. It forms HNO₃ and HNO₂ with water.
• B. Al₂O₃: This is amphoteric, not purely basic. It reacts with both acids and alkalis.
• C. SO₂: Sulphur is a non-metal, so this is an acidic oxide. It forms H₂SO₃ with water.
• D. CaO: Calcium is a Group 2 metal, so this is a basic oxide.

CaO + H₂O → Ca(OH)₂ (calcium hydroxide, an alkali)

CaO + 2HCl → CaCl₂ + H₂O (reacts with acid to form salt and water)

Step 4: Answer The basic oxide is CaO, which is option D.

Exam tip: Remember "Metals are Basic, Non-metals are Acidic" (MB, NA). The exception is the amphoteric group: Al, Zn, Pb, which you can recall as "Always Zealous People". If an oxide of one of these appears alongside a Group 1 or 2 metal oxide, choose the Group 1 or 2 oxide as the basic one.