Question

Green carbonate which turns black on heating is:
A.Copper carbonate
B.Sodium carbonate
C.Potassium carbonate
D.None of these

Answer 1

Hint:Metal carbonate on heating produces metal oxide along with liberation of carbon dioxide gas. It may be subject to change of color and can be determined easily on its completion.

Complete answer:
The thermal decomposition of metal carbonates produces metal oxide with the liberation of carbon dioxide gas. However, all metal carbonates do not produce carbon dioxide on even prolonged heating. This is due to the polarization of the electron cloud of the carbonate ion by the metal ion. The metal ion with more positive charge and small size like calcium, lead and copper ion are more likely to produce more polarization than the metal ion with single positive charge like sodium ion or potassium ion along with big size. Metal carbonate with greater polarization is more likely to decompose easily even at low temperature.
Metal carbonate like potassium carbonate and sodium carbonate do not undergo decomposition even after prolonged heating and thereby leads to no production of carbon dioxide gas. As the color of both the sodium carbonate and potassium carbonate is white and they do not undergo decomposition, the color remains the same after heating for such metal carbonate.
Metal carbonate like copper carbonate easily gets decomposed to produce metal oxide and carbon dioxide. The heating of copper carbonate to produce copper oxide and carbon dioxide can even be determined by observing the change of color of green copper carbonate to black colored copper oxide. Similarly, white colored lead carbonate on heating produces yellow colored lead oxide and liberates carbon dioxide gas easily.

Thus, the correct option is A.
Note:
Decomposition reactions are those reactions in which one compound is broken down to form more substance or we can say a single reactant breaks down to give simpler products known as decomposition reactions. Example of decomposition reaction are decomposition of calcium carbonate to calcium oxide and carbon dioxide \[\left( {{\text{CaC}}{{\text{O}}_{3\left( {\text{s}} \right)}} \to {\text{Ca}}{{\text{O}}_{\left( {\text{s}} \right)}} + {\text{C}}{{\text{O}}_{2\left( {\text{g}} \right)}}} \right)\] and heating of lead nitrate which causes emission of nitrogen dioxide \[\left( {2{\text{Pb}}{{\left( {{\text{N}}{{\text{O}}_3}} \right)}_2} \to 2{\text{Pb}}{{\text{O}}_{\left( {\text{s}} \right)}} + 4{\text{N}}{{\text{O}}_{2\left( {\text{g}} \right)}} + {{\text{O}}_{2\left( {\text{g}} \right)}}} \right)\] . These reactions are mostly endothermic reactions as decomposition heat is required.