Chapter 1: Chemical Reactions and Equations

Detailed Chapter Notes

A chemical reaction is basically a "before and after" story for matter — atoms rearrange themselves and something new is formed. You'll know a chemical reaction happened when you notice a change in colour, a gas bubbling up, a smell forming, a temperature shift, or a precipitate settling at the bottom.

A chemical equation is just the shorthand version of that story. Example: Mg + O2 → MgO. This is unbalanced — count the oxygen atoms, 2 on the left, 1 on the right. Balancing means the number of atoms of each element must be equal on both sides (Law of Conservation of Mass). Balanced version: 2Mg + O2 → 2MgO.

Combination reaction: two or more substances combine into one product. CaO + H2O → Ca(OH)2.

Decomposition reaction: one substance breaks into two or more — thermal (CaCO3 → CaO + CO2), electrolytic (water splitting into H2 and O2), or photolytic (AgCl breaking down in sunlight, the basis of black and white photography).

Displacement reaction: a more reactive element displaces a less reactive one from its compound. Fe + CuSO4 → FeSO4 + Cu.

Double displacement reaction: ions swap partners, usually forming a precipitate. Na2SO4 + BaCl2 → BaSO4 (white precipitate) + NaCl.

Oxidation and reduction (redox): oxidation = gain of oxygen/loss of hydrogen; reduction = loss of oxygen/gain of hydrogen. CuO + H2 → Cu + H2O — copper oxide is reduced, hydrogen is oxidised.

Corrosion is when a metal is slowly eaten away by reaction with air, moisture or acids — rusting of iron is the classic example. Rancidity is oxidation of fats/oils in food, which is why chip packets are flushed with nitrogen instead of normal air.