Chemical Reactions and Equations
Writing equations
A chemical equation shows reactants and products using symbols and formulae. It must be balanced, because mass is conserved: the number of atoms of each element is equal on both sides.
- Physical states: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution. An upward arrow shows a gas released; a downward arrow a precipitate.
- Example:
2H2 + O2 -> 2H2O. - A catalyst speeds up a reaction without being used up; enzymes are biological catalysts.
Types of chemical reactions
| Type | Description | Example |
|---|---|---|
| Combination | Two or more substances form one product | CaO + H2O -> Ca(OH)2 (slaking of lime); C + O2 -> CO2 |
| Decomposition | One substance breaks into simpler ones | CaCO3 -> CaO + CO2 (heat); 2H2O -> 2H2 + O2 (electricity) |
| Displacement | A more reactive element displaces a less reactive one | Fe + CuSO4 -> FeSO4 + Cu |
| Double displacement | Exchange of ions; often a precipitate | Na2SO4 + BaCl2 -> BaSO4 + 2NaCl |
| Oxidation | Gain of oxygen or loss of hydrogen / electrons | Burning of magnesium |
| Reduction | Loss of oxygen or gain of hydrogen / electrons | CuO + H2 -> Cu + H2O |
| Redox | Oxidation and reduction together | Most reactions above |
- Exothermic reactions release heat: burning of fuels, respiration, neutralisation, slaking of lime, decomposition of vegetable matter into compost.
- Endothermic reactions absorb heat: photosynthesis, decomposition of calcium carbonate, dissolving ammonium chloride.
- Thermal decomposition: by heat. Electrolysis: by electricity (water splits into hydrogen at the cathode and oxygen at the anode, in ratio 2 : 1). Photolysis: by light (silver chloride darkens, the basis of black-and-white photography).
- Precipitation reaction: an insoluble solid forms, e.g. barium sulphate (white), lead iodide (yellow).
- Oxidising agent gets reduced; reducing agent gets oxidised.
Corrosion and rancidity
- Corrosion is the slow destruction of metals by air and moisture. Rusting of iron needs both oxygen and water:
Fe2O3 . xH2O. Silver tarnishes (black silver sulphide) and copper turns green (basic copper carbonate). - Prevention of rusting: painting, oiling, galvanisation (zinc coating), tinning, electroplating, alloying (stainless steel), cathodic protection.
- Rancidity: oxidation of fats and oils in food, giving a bad smell and taste. Prevented by antioxidants, airtight containers, refrigeration, and flushing nitrogen in chips packets.
Keep practising
Notes are only half the work. Test yourself with questions and discuss doubts with other aspirants.
Prepared for exam revision from the NCERT Science textbooks for Classes 6 to 10 and standard sources. Space missions, awards and records change, so check official sources for recent facts. Found a mistake? Tell us.