Which one of the following oxides is a neutral oxide?
- (a)CO
- (b)CO2
- (c)Na2O
- (d)MgO
Answer
Why
Correct — A, CO. Carbon monoxide is a neutral oxide — neither acidic nor basic — and it does not form a salt with acids or bases under ordinary conditions. Carbon monoxide, along with water and nitrous oxide (N2O), makes up the classic set of neutral oxides.
Why the others are wrong
- (b)CO2 — Carbon dioxide is an acidic oxide: it dissolves in water to give carbonic acid and reacts with bases, for example turning lime water milky.
- (c)Na2O — Sodium oxide is a basic oxide of a metal; it dissolves in water to give the strong base sodium hydroxide.
- (d)MgO — Magnesium oxide is a basic oxide; it reacts with acids to form salts and gives an alkaline solution in water.
Concept
Oxides are classified by their acid-base behaviour: non-metal oxides are usually acidic (CO2, SO2), metal oxides are usually basic (Na2O, MgO), some are amphoteric (Al2O3, ZnO), and a few non-metal oxides are neutral (CO, N2O, water).
The trap pairs CO with CO2 — the same two elements but opposite behaviour. Carbon dioxide is acidic; only carbon monoxide is neutral, while the two metal oxides are basic.
Key facts
- Neutral oxides — CO, N2O and water — are neither acidic nor basic.
- CO2 and SO2 are acidic oxides that form acids with water.
- Na2O and MgO are basic oxides that form alkalis and react with acids.
- Al2O3 and ZnO are amphoteric — they react with both acids and bases.
Only CO is neutral, giving option (a).
Study next
Common traps
- Confusing CO (neutral) with CO2 (acidic).
- Assuming every carbon oxide is acidic — CO is the neutral exception.
A straightforward 'which oxide is neutral, acidic, basic or amphoteric' classification item.
Related PYQs
Carbon dioxide gas, when passed through lime water, brings about which of the following reactions?
- (a) CaCO3 + H2O + CO2 → Ca(HCO3)2
- (b) Ca(OH)2 + CO2 → CaCO3 + H2O
- (c) CaO + CO2 → CaCO3
- (d) CaCl2 + H2O + CO2 → CaCO3 + 2HCl
Answer(b) Ca(OH)2 + CO2 → CaCO3 + H2O
Reinforces the acid-base nature of oxides from the other side: CO2 is acidic and reacts with the basic lime water, which is exactly why CO2 is not the neutral oxide here — CO is.
Practice
- practice — not a real PYQ
Which of the following is an amphoteric oxide?
- (a)Na2O
- (b)CO2
- (c)Al2O3
- (d)CaO
Answer(c) Al2O3 — it reacts with both acids and bases. - practice — not a real PYQ
Which one of the following is an acidic oxide?
- (a)MgO
- (b)SO2
- (c)Na2O
- (d)CaO
Answer(b) SO2 — a non-metal oxide that forms sulphurous acid in water.