Q.Distinguish between oxidation and reduction.

Oxidation is the loss of an electron and reduction is the gain of an electron (the general definition, superseding the older oxygen/hydrogen-based one); the two always occur together in a redox reaction. Using oxidation-number change, every redox reaction can be classified into one of five types:

1. Combination reaction -- two substances combine into a single compound. Example: C + O₂ → CO₂ (C: 0→+4, oxidised; O: 0→−2, reduced).
2. Decomposition reaction -- the reverse: one compound breaks into two or more products, with different elements within it changing oxidation number in opposite directions. Example: 2KClO₃ → 2KCl + 3O₂ (Cl: +5→−1, reduced; O: −2→0, oxidised).
3. Displacement reaction -- an ion/atom in a compound is replaced by an ion/atom of another element. Metal displacement example: CuSO₄ + Zn → Cu + ZnSO₄ (Zn: 0→+2, oxidised; Cu: +2→0, reduced). Non-metal displacement example: Zn + 2HCl → ZnCl₂ + H₂ (Zn: 0→+2, oxidised; H: +1→0, reduced).
4. Disproportionation reaction (auto-redox) -- the same element in the same starting compound is simultaneously oxidised and reduced. Example: 2H₂O₂ → 2H₂O + O₂ (O: −1 splits into −2 in H₂O, reduced, and 0 in O₂, oxidised). …