What are acids, bases and pH?
Acids and bases are classes of substances defined by their reaction behavior. In the Brønsted–Lowry definition an acid donates a proton and a base accepts one. pH describes the acidity of an aqueous solution on a logarithmic scale based on hydrogen-ion activity.
Definitions and conjugate pairs
In water hydrochloric acid transfers a proton to water, producing hydronium; water acts as a base here. Ammonia accepts a proton from water and acts as a base. A proton donor and the species remaining after donation form a conjugate acid–base pair. “Strong” describes the extent of ionization in water, while “concentrated” describes amount per volume; these are different properties. A solution can contain a dilute strong acid or a concentrated weak acid. The Lewis definition offers a broader view based on electron-pair exchange.
pH and safe use
At about 25 °C, pH 7 is neutral for dilute aqueous solutions; lower values indicate acidity and higher values basicity. The scale is logarithmic: one pH unit represents roughly a tenfold difference in hydrogen-ion activity. The neutral point depends on temperature, and the familiar 0–14 range is not a rigid limit for highly concentrated solutions. Indicator paper gives approximate values; a pH meter requires calibration. Strong acids and bases may be corrosive and require appropriate laboratory protection.