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.

1. What are X, Y and Z?
2. Write equations for the reactions which take place:
   1. When limestone reacts with dilute HCl.
   2. When gas X reacts with lime water to form white precipitate Y.
   3. When excess of gas X dissolves white precipitate Y to form a soluble compound Z.

Answer 1

(a) X is carbon dioxide gas \[\ce{(CO2)}\].

Y is calcium carbonate \[\ce{(CaCO3)}\].

Z is calcium hydrogen carbonate (calcium bicarbonate) \[\ce{(Ca(HCO3)2)}\].

(b)

(i) When a piece of limestone or calcium carbonate is treated with dilute hydrochloric acid, carbon dioxide gas (\[\ce{CO2}\]_, here given as X) is produced.

\[\ce{CaCO3(s) + 2HCl(aq) → CaCl2(aq) +  H2O(l) + CO2(g)}\]

(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 (\[\ce{CaCO3}\], Here given as Y) is formed.

\[\ce{Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)}\]

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

\[\ce{CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq)}\]