Indicators and pH: compare aqueous solutions
Use litmus, thymolphthalein, methyl orange and universal indicator for the questions each can answer.
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Identify acid and alkali solutions through H⁺ and OH⁻.
- Distinguish an observation from a precise concentration measurement.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which aqueous sample is more acidic in the stated pH comparison?
What you will learn
- State the required indicator colours in acidic and alkaline solutions.
- Use universal indicator paper and pH to compare acidity, neutrality and alkalinity.
Remember the named indicator pairs
Litmus is red in acidic solution and blue in alkaline solution; purple is its neutral colour. Thymolphthalein is colourless in acidic solution and blue in sufficiently alkaline solution. Methyl orange is red in sufficiently acidic solution and yellow in alkaline solution, with orange in its transition range. Do not substitute phenolphthalein's pink colour for thymolphthalein's blue colour.
An indicator changes over its own pH interval, rather than giving a complete numerical pH measurement. Colourless thymolphthalein alone does not distinguish an acidic solution from a neutral one, because both can be below its blue transition. Methyl orange's orange transition must not be memorised as proof that every neutral solution is orange; near pH 7 it is yellow.
Use universal indicator for a wider comparison
Universal indicator paper is compared with its supplied colour chart to estimate pH. In the usual school aqueous comparison around room temperature, pH below 7 is acidic, about 7 neutral and above 7 alkaline. Typical chart colours move from red/orange/yellow for acidic conditions through green near neutral to blue/purple for alkaline conditions; match the specific product's chart rather than assigning one precise pH from a remembered shade.
A lower pH corresponds to higher hydrogen-ion concentration in the compared aqueous solutions, and a higher pH to lower hydrogen-ion concentration. Thus pH 2 is more acidic than pH 5, while pH 12 is more alkaline than pH 9. This is a relative comparison, not a claim that a pH 12 liquid is safe to touch or that an indicator can measure the total amount of dissolved acid. Strength and concentration are distinguished in the Extended lesson.
Pause and explain
A solution leaves thymolphthalein colourless. What can be concluded from that alone?
Worked example
Samples have pH 3, 7 and 11 using the supplied universal-indicator chart. Classify them and compare hydrogen-ion concentration.
Show the worked solution
- pH 3 is acidic, pH 7 approximately neutral and pH 11 alkaline in the stated conditions.
- The pH 3 sample has the highest hydrogen-ion concentration and pH 11 the lowest of these three.
- Use the chart for observed colours; classification does not itself establish acid concentration or handling safety.
Answer Acidic, neutral, alkaline respectively; hydrogen-ion concentration decreases from pH 3 to 7 to 11.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give acidic and alkaline colour pairs for litmus, thymolphthalein and methyl orange.
Give me a hint
Use the named indicator, rather than another indicator with a similar name.
Compare my reasoning
- Litmus: acidic red, alkaline blue.
- Thymolphthalein: acidic colourless, sufficiently alkaline blue.
- Methyl orange: sufficiently acidic red, alkaline yellow.
Litmus red/blue; thymolphthalein colourless/blue; methyl orange red/yellow.
Look for these in your work
- Each pair stays with its named indicator.
- Transition-range limitations are retained.
Order pH 2, 5, 9 and 12 by increasing hydrogen-ion concentration, and identify the most alkaline sample.
Give me a hint
Hydrogen-ion concentration decreases as pH increases.
Compare my reasoning
- The lowest hydrogen-ion concentration is at pH 12.
- Increasing concentration order is pH 12, 9, 5, 2.
- pH 12 is the most alkaline of these stated samples.
Increasing H⁺ concentration: pH 12, 9, 5, 2. Most alkaline: pH 12.
Look for these in your work
- The ordering uses the correct direction.
- Relative alkalinity is identified consistently.
A student labels a colourless thymolphthalein sample acidic and a yellow methyl-orange sample exactly pH 7. Explain both overclaims and choose a better approximate pH measurement.
Give me a hint
One colour can cover a range; use a wider indicator chart for an estimate.
Compare my reasoning
- Colourless thymolphthalein can include acidic and neutral samples.
- Yellow methyl orange covers multiple pH values and does not fix pH at 7.
- Universal indicator paper with its supplied chart gives a more useful approximate pH comparison.
Neither colour fixes the claimed classification or exact pH; use universal indicator with its chart for an estimate.
Look for these in your work
- Both overclaims are explained.
- The alternative measurement matches the question.
Common mistakes
- Using pink as the alkaline thymolphthalein colour.
- Treating colourless thymolphthalein as proof of acidity.
- Calling orange methyl orange a universal neutral-pH result.
- Calling a larger pH more acidic.
What can you explain now?
State thymolphthalein's acidic and sufficiently alkaline colours.
Compare with the explanation
Acidic: colourless. Sufficiently alkaline: blue.
Thymolphthalein uses the colourless/blue pair; it is not phenolphthalein's colourless/pink pair.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recall all three named pairs, then explain why an indicator colour is not always a unique pH. Rank a fresh set of pH values by hydrogen-ion concentration.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.