Question

When a piece of limestone reacts with dilute HCl, a gas X is produced. When gas X is passed through lime water then a white precipitate Y is formed. On passing excess of gas X, the white precipitate dissolves forming a soluble compound Z.
(a) What are X, Y and Z?
(b) Write equations for the reactions which take place:
(i) when limestone reacts with dilute HCl
(ii) when gas X reacts with lime water to form white precipitate Y
(iii) when excess of gas X dissolves white precipitate Y to form a soluble compound Z

Answer 1

(a) X is carbon dioxide gas (CO₂).
Y is calcium carbonate (CaCO₃).
Z is calcium hydrogen carbonate (calcium bicarbonate) [Ca(HCO₃)₂].

(b)
(i) When a piece of limestone or calcium carbonate is treated with dilute hydrochloric acid, carbon dioxide gas (CO_{2 ,} here given as X) is produced.
CaCO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

(ii) When carbon dioxide (X), liberated in the reaction is passed through lime water, the lime water turns milky or a white precipitate of calcium carbonate (CaCO₃ , Here given as Y) is formed.

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

(iii) When excess of carbon dioxide is passed through lime water, a solution of calcium hydrogen carbonate [Z, Ca(HCO₃)₂] is formed.

CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)