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

1. X is carbon dioxide gas (CO₂).
   Y is calcium carbonate (CaCO₃).
   Z is calcium hydrogen carbonate (calcium bicarbonate) (Ca(HCO₃)₂).
   1. When a piece of limestone or calcium carbonate is treated with dilute hydrochloric acid, carbon dioxide gas (CO₂, here given as X) is produced.
      \[\ce{CaCO3_{(s)} + 2HCl_{(aq)} -> CaCl2_{(aq)} +  H2O_{(l)} + CO2_{(g)}}\]
   2. 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.
      \[\ce{Ca(OH)2_{(aq)} + CO2_{(g)} -> CaCO3_{(s)} + H2O_{(l)}}\]
   3. When an excess of carbon dioxide is passed through lime water, a solution of calcium hydrogen carbonate [Z, Ca(HCO₃)₂] is formed.
      \[\ce{CaCO3_{(s)} + H2O_{(l)} + CO2_{(g)} -> Ca(HCO3)2_{(aq)}}\]