Chemical reactions, acids, bases and salts

A chemical reaction rearranges atoms into new substances: bonds break, atoms regroup, and new bonds form. Unlike a physical change such as melting ice, a chemical change produces substances with different properties — burning wood gives ash, smoke, and gases that cannot be turned back into wood. A fundamental rule governs all reactions: atoms are neither created nor destroyed, only rearranged. That is why chemical equations must balance: the number of atoms of each element is the same on both sides. The French chemist Antoine Lavoisier established this conservation of mass in the eighteenth century, laying the foundation of modern chemistry.

Reactions often involve energy. Burning fuel releases heat and light — an exothermic reaction — while cooking an egg or photosynthesis absorbs energy — endothermic processes. The rate of a reaction depends on temperature, concentration, surface area, and catalysts, substances that speed a reaction without being consumed. A catalyst in a car's exhaust system, for instance, converts harmful gases into less harmful ones without being used up. Enzymes are biological catalysts, and life depends on them: without enzymes, the reactions in our cells would run far too slowly to sustain us.

Acids and bases are two important classes of substances. Acids taste sour, react with metals, and turn blue litmus paper red; familiar examples include citric acid in lemons and hydrochloric acid in stomach fluid. Bases, or alkalis when soluble, feel slippery and turn red litmus blue; soap and baking soda are common examples. The pH scale measures acidity from 0 to 14, with 7 neutral: below 7 is acidic, above 7 is basic. Each step on the scale is a tenfold change in acidity, so a liquid at pH 3 is a hundred times more acidic than one at pH 5.

When an acid reacts with a base, they neutralise each other to form a salt and water. Table salt, sodium chloride, is one such salt, but the class is large: calcium carbonate forms chalk and limestone, and sodium bicarbonate is baking soda. Oxidation is another central reaction type: the rusting of iron, the browning of a cut apple, and the burning of fuel are all oxidations, involving oxygen or the loss of electrons. Recognising these patterns turns everyday observations — sour milk, tarnished silver, a fizzing antacid tablet — into chemistry in action.