JEE Main 2021 · 26 February · evening · Single correct · +4 / −1
Which pair of oxides is acidic in nature?
- (a) CaO, SiO₂
- (b) B₂O₃, CaO
- (c) B₂O₃, SiO₂
- (d) N₂O, BaO
Answer and solution
Answer (c) B₂O₃, SiO₂
Solution
Chemistry ma’am · 3 steps
Look at the element behind the oxygen: non-metals give acidic oxides, metals like Ca and Ba give basic ones.
- The rule. Oxides of metals, especially the alkali and alkaline earth metals, are basic. Oxides of non-metals are usually acidic. A few low-oxidation-state non-metal oxides, such as N₂O, NO and CO, are neutral.
Oxide Element type With water Nature CaO metal (group 2) gives Ca(OH)₂ basic BaO metal (group 2) gives Ba(OH)₂ basic N₂O non-metal, N in +1 state no acid formed neutral B₂O₃ non-metal (group 13) gives boric acid, H₃BO₃ acidic SiO₂ non-metal (group 14) insoluble, but reacts with NaOH to give Na₂SiO₃ acidic - Why SiO2 counts. SiO₂ does not dissolve in water, so judge it by what it reacts with. It reacts with bases to form silicates, which makes it an acidic oxide.
- So. Only (c) has two acidic oxides: B₂O₃ and SiO₂. Every other option pairs a basic or neutral oxide with something.
The idea to keep. Find the element first. Metal oxides are basic, non-metal oxides are acidic, and N₂O, NO, CO are the neutral exceptions to remember.
Answer from the NTA final answer key. Solution by PrepWiser.