Universal Indicator – IGCSE Chemistry Definition and Key Facts

Universal indicator is tested in the Cambridge 0620 syllabus as the standard method for estimating pH. It provides more information than litmus — instead of just “acid or alkali,” it gives an approximate pH value to the nearest whole number.

The 0620 definition

Universal indicator is a mixture of several different indicators that produces a continuous range of colours depending on the pH of the solution being tested. It is available as a solution or as pH paper (paper soaked in universal indicator and dried).

The colour scale

pH	Colour	Classification
1	Red	Strong acid
2	Red/orange	Strong acid
3	Orange	Acid
4	Orange/yellow	Weak acid
5	Yellow	Weak acid
6	Yellow/green	Very weak acid
7	Green	Neutral
8	Blue/green	Very weak alkali
9	Blue	Weak alkali
10	Blue	Weak alkali
11	Dark blue/indigo	Alkali
12	Violet/purple	Strong alkali
13	Purple	Strong alkali
14	Dark purple	Strong alkali

The key colours to remember: red = strong acid, green = neutral, purple = strong alkali.

How to use universal indicator

Solution form

Add 2-3 drops of universal indicator solution to a sample of the test solution in a test tube or on a spotting tile. Compare the colour produced with the pH colour chart.

pH paper form

Dip a strip of pH paper into the solution (or place a drop of solution on the paper using a glass rod). Compare the resulting colour with the chart. Do not dip the pH paper into the stock bottle — use a separate sample to avoid contamination.

Universal indicator vs litmus

Feature	Litmus	Universal indicator
Colours produced	Red or blue (or purple)	Full spectrum from red to purple
Information given	Acid or alkali only	Approximate pH value
Can distinguish strong from weak acid?	No	Yes (different shades of red/orange)
Can identify neutral solutions?	Limited (purple)	Yes (green)
Suitable for titrations?	No	No

Universal indicator vs pH meter

Feature	Universal indicator	pH meter
Precision	Nearest whole pH unit	To 0.01 pH units
Cost	Very low	Higher (electronic equipment)
Ease of use	Simple — just compare colours	Needs calibration
Quantitative?	Semi-quantitative	Fully quantitative
Best for	Classroom estimates	Accurate measurements, titrations

For exam purposes: if the question asks for an “accurate” or “precise” pH measurement, suggest a pH meter. If it asks for a quick estimate, universal indicator is sufficient.

Why universal indicator works

Universal indicator contains several different indicators, each of which changes colour over a different pH range. By combining them, the mixture produces a smooth transition of colours across the full pH range rather than the single colour change of one indicator.

Strong vs weak acids with universal indicator (Supplement)

Solution	Concentration	pH	Colour
Hydrochloric acid (strong)	0.1 mol/dm³	~1	Red
Ethanoic acid (weak)	0.1 mol/dm³	~3	Orange

Both are acids at the same concentration, but the strong acid has a lower pH because it is fully ionised, producing more H⁺ ions.

Worked exam question

A student adds universal indicator to four solutions and records these colours: A = red, B = green, C = yellow, D = purple. (a) Which solution is neutral? [1] (b) Which solution has the lowest pH? [1] (c) State the approximate pH of solution C. [1]

(a) B (green = pH 7 = neutral) [1]

(b) A (red = pH 1-2 = lowest) [1]

(c) pH 5 or 6 (yellow = weakly acidic) [1]

Common exam mistakes

1. Saying neutral solutions turn litmus green — green is the neutral colour of universal indicator, not litmus. Litmus is purple when neutral.
2. Giving a specific pH like 3.5 from universal indicator — it only gives whole-number estimates. For decimal accuracy, a pH meter is needed.
3. Using universal indicator in a titration — it changes colour gradually and cannot identify the sharp endpoint needed.
4. Confusing the indicator colour with the solution colour. Always compare the indicator colour with the pH chart, not the original colour of the solution.