Decomposition reactions are those in which a compound breaks down into simpler compounds. For example, on heating, calcium carbonate decomposes to give calcium oxide and carbon dioxide.
CaCO3(s) CaO(s) + CO2(g)
Water decomposes to give hydrogen and oxygen when electricity is passed through it.
2H2O(l) 2H2(g) + O2(g)
Take a piece of a marble. Hold it with a pair of tongs. Heat it over flame for sometime. Let it cool. Drop the solid into water. Add a drop of red litmus to the solution. The colour of litmus changes to blue.
Calcium carbonate (marble) decomposes to calcium oxide and carbon dioxide. The calcium oxide formed, reacts with water to give an alkaline solution. Therefore, red litmus turns blue.
Take 0.5 g of zinc carbonate in a test tube. Fit the test tube with a cork and a delivery tube. Heat over the flame. A colourless gas is given out. The gas on passing through lime water, turns it milky.
Heating zinc carbonate decomposes it to carbon dioxide gas and zinc oxide. The carbon dioxide turns lime water milky.
ZnCO3(s) ZnO(s) + CO2(g)
Heat about 1.0g of ferrous sulphate in a china dish. On strong heating the green colour of ferrous sulphate changes to brown. A pungent-smelling gas also comes out of it. This is due to the decomposition of ferrous sulphate to give brown ferric oxide and pungent gases like SO2 and SO3.