The reagent used in testing for carbon (IV) oxide is

Step 1: Analyze the question You're asked which reagent is used to test for carbon (IV) oxide (CO₂).

Step 2: The concept The standard laboratory test for CO₂ uses lime water, which is a solution of calcium hydroxide, Ca(OH)₂. When CO₂ is bubbled through it, the clear solution turns milky (cloudy) because insoluble calcium carbonate forms:

Ca(OH)₂ + CO₂ → CaCO₃ (white precipitate) + H₂O

If you keep passing CO₂ through, the milkiness disappears. This is because soluble calcium hydrogencarbonate forms:

CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂

Step 3: Check each option

• A. Copper sulphate solution: used to test for water (anhydrous copper(II) sulphate turns from white to blue) or in Benedict's and Biuret tests. It does not test for CO₂.
• B. Lime water: turns milky with CO₂. ✔
• C. Hydrochloric acid: it is used to produce CO₂ from carbonates, not to detect it.
• D. Sodium hydroxide solution: it absorbs CO₂, but it gives no visible change, so it is not a test. In biology it is used to remove CO₂ in experiments.

Step 4: Answer The correct option is B. lime water.

Exam tip: Remember "Lime water = Limits CO₂": lime water goes milky for CO₂. In biology practicals, such as respiration or photosynthesis experiments, lime water shows CO₂ was given out, while sodium hydroxide or potassium hydroxide is used to absorb CO₂. Don't mix up the two.