What is a decomposition reaction? Describe thermal, electrolytic and photolytic decomposition with one example each.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define in one line. In a decomposition reaction, a single reactant breaks down into two or more simpler products.
2 · Thermal decomposition. Heat breaks the reactant: CaCO₃(s) → CaO(s) + CO₂(g). Heating ferrous sulphate is another example: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃.
3 · Electrolytic decomposition. An electric current breaks water: 2H₂O(l) → 2H₂(g) + O₂(g). The hydrogen volume is double.
4 · Draw the electrolysis set-up. Draw a plastic mug with two carbon electrodes fixed through its base. Cover each electrode with an upside-down test tube full of water, and join the electrodes to a 6 V battery. Label: cathode, anode, hydrogen, oxygen, water with a few drops of dilute sulphuric acid.
5 · Photolytic decomposition. Sunlight breaks silver chloride: 2AgCl(s) → 2Ag(s) + Cl₂(g). The white solid turns grey.
6 · Link to real life. Quicklime for cement comes from heating limestone; silver salts were used in black and white photography.
7 · Close in one line. All three need energy, so decomposition reactions are usually endothermic.
Scoring tip: Write one balanced equation for each kind and name the energy used. A neat electrolysis diagram earns an extra mark.
