The reaction of hydrochloric acid and calcium trioxocarbonate (IV) produces

Step 1: Analyze the question We need the products when hydrochloric acid (HCl) reacts with calcium trioxocarbonate (IV), which is calcium carbonate (CaCO₃), a carbonate.

Step 2: The concept Acids react with carbonates to form a salt + carbon (IV) oxide + water.

This happens because the carbonic acid (trioxocarbonate (IV) acid, H₂CO₃) formed in the first step is unstable. It breaks down immediately into CO₂ and H₂O.

Step 3: Solution

Step (i): Double displacement CaCO₃ + 2HCl → CaCl₂ + H₂CO₃

Step (ii): Decomposition of the unstable acid H₂CO₃ → H₂O + CO₂

Overall equation: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Step 4: Check the options

• A. Calcium chloride, CO₂ and water: matches the overall equation.
• B. Carbonic acid is unstable and does not remain as a product.
• C. No calcium hydroxide or chlorine gas is formed.
• D. The salt formed is CaCl₂, not Ca(OH)₂.
• E. Calcium metal is never produced in a simple acid-carbonate reaction.

Answer: A

Exam tip: Remember "Acid + Carbonate → Salt + Water + CO₂". The fizzing (effervescence) you see in the lab is CO₂ escaping. It turns lime water milky, which is the standard test for CO₂.