Easy
A few drops of dilute hydrochloric acid are added to sodium carbonate. A gas is evolved that turns lime water milky. The gas is:
Explanation
Explanation
Step 1: Write the reaction: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂.
Step 2: Acids react with metal carbonates to give salt + water + carbon dioxide (bubbles of brisk effervescence).
Step 3: Lime water test: Ca(OH)₂ + CO₂ → CaCO₃ (white, milky) + H₂O. Only CO₂ gives this result.
Step 4: Hydrogen burns with a pop sound, and oxygen relights a glowing splinter. They do not turn lime water milky.
Answer: Carbon dioxide.
PRO TIP
Re-check your working by substituting the answer back into the original question.