To classify a solution from an indicator, compare the colour with the range where that indicator changes, then decide only what the colour proves. Every indicator answers one limited question, and a good answer says no more than the result allows. Indicator work appears in practical questions, in titration questions and in short “state what the result shows” items.
This lesson sits at the start of acids, bases and salts.
What does a colour actually tell you?
An indicator is a substance whose colour depends on whether the solution is acidic or alkaline. The solution is called acidic below pH 7, neutral at pH 7 and alkaline above pH 7. Some indicators give a single switch, and universal indicator gives a range of colours that match the pH scale.
| Indicator | Acidic | Neutral | Alkaline |
|---|---|---|---|
| Litmus | red | purple | blue |
| Methyl orange | red | yellow | yellow |
| Phenolphthalein | colourless | colourless | pink |
| Universal indicator | red, orange, yellow | green | blue, purple |
The table shows common school colours. Check the colours your syllabus and practical paper use, because some papers supply their own chart.
How do you decide from a result?
- Name the indicator and recall its two or three colours.
- Read the colour exactly as supplied, for example “orange” rather than “red-ish”.
- Match it to a class (acidic, neutral, alkaline) only if the colour belongs to one class.
- If the colour fits more than one class, say what you can and cannot conclude.
- Use the pH number when one is given. A value below 7 is acidic and above 7 is alkaline. The scale also shows strength, because pH 1 is far more acidic than pH 6.
Worked example
The results below are invented for teaching. Classify each solution.
| Solution | Indicator | Colour |
|---|---|---|
| A | Universal indicator | red |
| B | Litmus | blue |
| C | Phenolphthalein | colourless |
| D | Methyl orange | yellow |
| E | Universal indicator | green |
A: red on universal indicator is at the very acidic end. A is strongly acidic.
B: blue litmus means alkaline.
C: colourless phenolphthalein is seen in acid and in neutral solutions. C is acidic or neutral, and another test would be needed to decide.
D: yellow methyl orange appears in neutral and alkaline solutions. D is neutral or alkaline, and a second indicator would separate them.
E: green on universal indicator matches pH 7, so E is neutral.
The mistake to watch for
Mistaken answer: “C is colourless with phenolphthalein, so C is an acid.”
The student remembered that phenolphthalein is colourless in acid and stopped there.
The correction is to ask the reverse question: “what else is colourless?” Phenolphthalein is also colourless in a neutral solution. The accurate statement is that C is not alkaline. When a question asks you to “classify”, look for the indicator that can separate the classes, or give both possibilities.
Check yourself
1. Litmus is red in solution F and purple in solution G. Classify each.
Show answer
Red litmus means F is acidic. Purple litmus means G is neutral.
2. A student tests a solution with universal indicator and records “orange, pH 4”. Classify it and say whether it is very acidic.
Show answer
pH 4 is below 7, so the solution is acidic. It is only mildly acidic compared with pH 1 or 2.
3. Solution H turns phenolphthalein pink and solution J turns methyl orange yellow. Which one can you classify as alkaline, and why?
Show answer
H is alkaline, because pink phenolphthalein belongs only to alkaline solutions. Yellow methyl orange fits neutral and alkaline, so J cannot be classified with certainty.
Where this leads next
The next lesson explains why these colours change by looking at the particles behind acidity. You can then test yourself on the acids, bases and salts practice set. The mole and equation-ratio tutor and the equation balance reasoning trainer help when the same topic turns into equations.
If the trouble is deciding what a result proves rather than remembering the colours, that is the kind of reasoning our teachers look at in online one-to-one Chemistry tuition.