Which gas turns lime water milky?

**The reasoning** When carbon dioxide (CO₂) gas is bubbled through lime water (a clear solution of calcium hydroxide, Ca(OH)₂), a chemical reaction occurs: **CO₂ + Ca(OH)₂ → CaCO₃ + H₂O** The product, calcium carbonate (CaCO₃), is **insoluble** — it forms tiny white particles suspended in the water, making it appear milky or cloudy. This is a classic test for CO₂ in the lab. Think of it like chalk dust in water! **Why the wrong options tempt you** - **Oxygen** is what we breathe and supports burning, but it's chemically unreactive with lime water. - **Nitrogen** makes up 78% of air but is extremely inert — it won't react with lime water at all. - **Hydrogen** burns with a "pop" sound (that's its test!), but doesn't interact with lime water either. These gases might sound scientific, but only CO₂ produces that telltale milky precipitate. **Quick takeaway** Carbon dioxide turns lime water milky because it forms insoluble calcium carbonate — this is THE definitive test for CO₂ you'll see in WAEC/JAMB practicals!