Common alkali:

**The reasoning** An **alkali** is a base that dissolves in water to produce hydroxide ions (OH⁻). The key word here is "alkali" — not just any base, but specifically one that's water-soluble. NaOH (Sodium hydroxide) is the classic example. When it dissolves: NaOH → Na⁺ + OH⁻ It's called "caustic soda" in everyday language and turns red litmus paper blue. It's found in soap-making and drain cleaners. **Why the wrong options tempt you** **HCl** (Hydrochloric acid) and **H₂SO₄** (Sulfuric acid) are acids, not alkalis — they produce H⁺ ions, not OH⁻. The question clearly says "alkali," so these are opposite. **CO₂** (Carbon dioxide) is tricky because it forms a weak acid (carbonic acid) when dissolved in water, making it acidic, not alkaline. **Quick takeaway** Remember: **Alkalis are bases with OH⁻** — if you see hydroxide (like NaOH, KOH, Ca(OH)₂), it's an alkali; if you see H at the front (HCl, H₂SO₄), it's an acid!