Acids, bases and the pH scale

The simplest definition says an acid releases hydrogen ions in water and a base releases hydroxide ions. The broader Bronsted-Lowry definition says an acid donates a proton and a base accepts one, which handles reactions outside water and explains why ammonia is a base despite containing no hydroxide.

The broadest definition, from Lewis, describes an acid as an electron pair acceptor and a base as a donor. For most school-level work the Bronsted-Lowry view is the right tool: it makes conjugate pairs obvious, since removing a proton from an acid leaves its conjugate base.

Strength describes how completely an acid dissociates. A strong acid such as hydrochloric acid ionises essentially completely; a weak acid such as ethanoic acid reaches an equilibrium with most molecules intact.

Concentration describes how much acid is dissolved per unit volume. A dilute strong acid and a concentrated weak acid are entirely different things, and exam questions exploit the confusion regularly.

pH is the negative logarithm of the hydrogen ion concentration. Because it is logarithmic, each whole step is a tenfold change: pH 3 is ten times more acidic than pH 4 and a hundred times more than pH 5.

Pure water at room temperature sits at 7 because it self-ionises slightly, producing equal small amounts of hydrogen and hydroxide ions. Below 7 is acidic, above is basic. The scale is not strictly capped at 0 and 14; those are simply the range of common solutions.

An acid and a base react to form a salt and water. Titration measures concentration by adding one solution to a known volume of the other until an indicator changes colour at the endpoint, then using the mole ratio from the balanced equation.

Indicator choice matters because the equivalence point is not always pH 7. A strong acid with a weak base ends acidic, a weak acid with a strong base ends basic, and the indicator must change colour in the steep region of that particular curve.

A buffer resists pH change when small amounts of acid or base are added. It is made from a weak acid together with its conjugate base, and it works because one component neutralises added base while the other neutralises added acid.

Buffers are why blood stays near pH 7.4 despite constant metabolic acid production. The carbonic acid and bicarbonate system does this work, and its failure in either direction, acidosis or alkalosis, is medically serious. This is a good example of chemistry and biology answering the same question.