Indicators, Strong Acids and Strong Alkalis

Explained

Indicators, and what the pH scale is really telling you

Three indicators are examinable, and each gives a simple two colour answer. Universal indicator is different: it gives a range of colours and therefore an approximate pH rather than a yes or no. Knowing which type a question is asking about decides how you should answer it.

The three you must know

• Litmus: red in acid, blue in alkali.
• Phenolphthalein: colourless in acid, pink in alkali.
• Methyl orange: red in acid, yellow in alkali. Near the changeover it looks orange.

Phenolphthalein catches people out because colourless is a result, not an absence of one. Writing that "nothing happened" loses the mark. The correct answer is that the indicator stayed colourless, which tells you the solution is acidic.

Universal indicator and the pH scale

Universal indicator is a mixture, which is why it produces a continuous range of colours rather than one change. Red is strongly acidic, orange and yellow weakly acidic, green neutral at pH 7, blue weakly alkaline and purple strongly alkaline.

Because the colour is matched to a chart by eye, it gives an approximate value. A pH meter gives a precise one. If a question asks how to improve accuracy when measuring pH, that is usually the answer being looked for.

Strong and weak are not the same as concentrated and dilute

This is the distinction that separates the top answers from the rest. A strong acid is one that fully dissociates in water, releasing all of its hydrogen ions. A weak acid only partly dissociates, so at the same concentration it releases fewer hydrogen ions and has a higher pH.

Concentration is about how much acid there is in a given volume. Strength is about how completely that acid splits up. A dilute strong acid and a concentrated weak acid are entirely different things, and questions do rely on the difference.

Reading the bands

As a rough guide, strong acids sit around pH 1 to 2, weak acids around 3 to 6, pure water at 7, weak alkalis around 8 to 11 and strong alkalis around 12 to 14.

If you are given a pH and asked whether the substance is a strong or weak acid, the band is your evidence. Say the value, say which band it falls in, and then say what that means about dissociation. Stating the band alone rarely gets the second mark.