Chemical Reactions, Acids, and Bases
Chemical reactions
A chemical reaction produces new substances with different properties from the reactants. Signs include gas production, colour change, temperature change, light, and precipitate formation. Word equations show reactants → products; chemical equations must be balanced to conserve atoms. Key reaction types: combustion (fuel + oxygen → carbon dioxide + water), oxidation, displacement (more reactive metal replaces less reactive), neutralisation (acid + base → salt + water), and thermal decomposition. The reactivity series predicts displacement; metals above hydrogen react with acids to release hydrogen gas.
Acids and bases
Acids have pH below 7, release hydrogen ions in solution, turn blue litmus red, and react with metals and carbonates. Bases have pH above 7, turn red litmus blue; alkalis are soluble bases. The pH scale runs 0–14; each step represents a tenfold change in hydrogen ion concentration. Indicators (litmus, universal indicator, methyl orange, phenolphthalein) show acidity. Neutralisation produces a salt and water; the salt name depends on the acid (hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate).
Rates of reaction
Rate increases with higher temperature, higher concentration, larger surface area, and catalysts. Collision theory explains this: more frequent and more energetic collisions between particles. Traps: confusing rate with yield - a catalyst speeds up a reaction but does not change the final amount of product.
Worked example
Sodium hydroxide + hydrochloric acid → sodium chloride + water. This is a neutralisation. If universal indicator is added, the solution starts blue (base) and turns green at neutral pH 7.