Acids, Bases and the pH Scale

An acid releases hydrogen ions (H+) when dissolved in water, for example HCl -> H+ + Cl-. A base does the opposite: it releases hydroxide ions (OH-), like NaOH -> Na+ + OH-, or it accepts an H+, like ammonia. A base that dissolves in water is called an alkali.

pH measures how acidic or alkaline a solution is, from 0 to 14. The more H+ ions, the lower the pH. Each step of 1 means ten times more (or fewer) H+ ions, so pH 3 has 10000 times more H+ than pH 7. Universal indicator shows the whole range as colours; a pH meter gives a number.

Strong acids (HCl, H2SO4) split completely into ions in water. Weak acids (ethanoic acid, CH3COOH) only partly split, sitting in an equilibrium. Strength is about how fully it ionises, not how concentrated it is. A dilute strong acid can still be more corrosive than a concentrated weak one.

When an acid meets a base they cancel out to give a salt and water: acid + base -> salt + water. At ion level it is always H+ + OH- -> H2O. The acid decides the salt: HCl gives chlorides, H2SO4 gives sulfates, HNO3 gives nitrates. The reaction gives out heat (exothermic).