Question

State the conditions necessary for a chemical change or reaction.

Answer 1

1. Mixing (close contact): In some cases, a chemical reaction occurs when two substances are mixed in their solid states. For example., Iodine and phosphorus react explosively when brought into close contact.
2. Solution: In some cases, a chemical reaction occurs when substances are mixed in either a molten or aqueous state. For example., Oxalic acid crystals and sodium carbonate react in water solution only.
3. Heat: Some chemical reactions occur only on heating. For example., Copper carbonate decomposes on heating (Δ symbol for heating) into copper oxide and carbon dioxide. 
   \[\ce{CuCO3_{(s)} ->[\Delta] CuO_{(s)} + CO2_{(g)}}\]
4. Light: Some chemical reactions take place by the action of light. These are called photochemical reactions or photolysis. Molecules of the reactants absorb light energy to get activated and then react rapidly. For example., Plants form glucose from carbon dioxide and water in the presence of light.
   \[\ce{6CO2 + 12H2O ->[Light] C6H12O6 + CO2 + 6H2O}\]
5. Electricity: Certain chemical reactions, like the decomposition of certain compounds, occur only when electricity is passed through the substance. For example., Electrolysis of acidulated water occurs only in the presence of electricity, to give hydrogen and oxygen.
   \[\ce{2H2O ->[electricity] 2H2 + O2}\]
6. Pressure: Some chemical reactions take place only when the involved substances are subjected to high pressure. For example., Mercuric chloride and potassium iodide when rubbed in a mortar, give a scarlet-coloured substance called mercuric iodide.
   \[\ce{HgCl2 + 2KI -> HgI2 + KCl}\]
7. Catalyst: Some chemical reactions need a catalyst to accelerate or decelerate the rate at which they occur. The catalysts themselves do not take part in the reaction and so remain unchanged. For example., Ammonia reacts with oxygen to produce nitric oxide and water vapour in the presence of (platinum) a catalyst.
   \[\ce{4NH3 + 5O2 ->[Pt][800° C] 4No + 6H2O}\]