General Chemistry · Acid–Base Equilibria · ACID-BASE INDICATORS
Acid–base indicators and their transition ranges
Change the pH and watch how the colors of seven indicators change.
What can one indicator color tell you about the pH of a solution? What changes when you use several indicators?
Learning objectives After this activity you can …
Estimated time 15 min· Prerequisites: the pH scale (0–14); acidic, neutral and basic solutions
explain that an indicator changes color only inside its own transition range of about 2 pH units.
infer the possible pH range from an observed color.
combine several indicators to narrow down the pH range of an unknown solution.
Bar = transition range (the pH range where the color changes gradually). Words beside each bar = acid-form color / base-form color (written as text, so you do not need to rely on color). Click a row (mouse) or use the checkbox list on the right (keyboard) to add an indicator.
The possible pH range is shown after you submit your prediction.
Entry check Before you start — prerequisites and key terms
What is an acid–base indicator?
A weak-acid (or weak-base) dye whose acid form and base form have different colorsA substance that reacts and forms a color only when an acid is presentA substance that brings a solution to neutral (pH 7)
Key terms
Indicator (HIn): a weak-acid dye whose acid form HIn and base form In⁻ have different colors. Equilibrium: HIn ⇌ H⁺ + In⁻. ·
Transition range: the pH range where both forms are present and the color changes gradually (usually about 2 pH units). ·
Possible pH range: all pH values that agree with the observed color. ·
pH: −log[H⁺]. At 25 °C, below 7 is acidic, 7 is neutral, above 7 is basic.
Predict Think first, before you observe
You add phenolphthalein to a solution and it stays colorless. What can you say about this solution for certain?
It is acidic (pH is lower than 7)Its pH is lower than about 8.2 (we only know a range)It is neutral (pH is exactly 7)
ConfidenceMedium
Observation task: in explore mode, add phenolphthalein only and move the slider to pH 7.0–8.1 (weakly basic). The solution is basic — what color do you see?
The explanation opens after you lock in a prediction
Why the color changes only inside a "range"
An indicator is a weak acid, HIn. In HIn ⇌ H⁺ + In⁻, HIn (acid-form color) dominates at low pH and In⁻ (base-form color) dominates at high pH.
Our eyes see the color of one form when that form is about 10 times more abundant than the other. So the color changes only within about pKa ± 1, that is, within about 2 pH units.
Outside this range, the color stays the same however much the pH changes. Example: methyl red is red at pH 4.5 and below, turns gradually yellow from 4.5 to 6.0, and is yellow at 6.0 and above.
One color = one pH range
Colorless phenolphthalein only means "pH is below about 8.2". A solution at pH 3 (acidic), pH 7 (neutral) or pH 7.8 (weakly basic) is colorless in every case.
So colorless ≠ acidic. To narrow the range, use indicators with different transition ranges together. This is why pH paper is coated with a mixture of several indicators. Soil pH test kits use the same idea.
Limits of this model
The transition-range limits are approximate values (±0.3) taken from standard literature tables. They change with temperature, ionic strength, indicator concentration and the observer.
The screen colors are approximations and may differ from real colors. Base your decision on the color names written as text.
The limits are not sharp lines. The model simplifies "outside the range = color of one form". Do not read results such as a 0.1-unit gap between two indicator limits as real precision. Phenolphthalein slowly becomes colorless again in very strong base (pH above about 13); this model does not include that.
Key summary
An indicator changes color only inside its own transition range of about 2 pH units.
What an observed color tells you (a value or a range?) is shown after you submit your prediction.An observed color gives a pH range, not a pH value (colorless phenolphthalein → pH < about 8.2).
Combining indicators with different transition ranges narrows the possible range (the overlap = the intersection).
Explanation cards
The cards "Why the color changes only inside a range" and "One color = one pH range", and the second summary line, open after you submit your prediction.
Check Formative check — pass all items to complete
Q1A solution with bromthymol blue is green (between yellow and blue). Which pH fits this solution best?
Between about 6.0 and 7.6Higher than 7.6Exactly 7.0
Hint
An in-between color means the acid form and the base form are both present. In which pH range does this happen?
Q2 · TaskSwitch to unknown sample mode (the pH is hidden). Add indicators and narrow the width of the possible pH range to 1.5 pH units or less.
Current range width: — · Indicators tried: 0 (at least 2) · Target width ≤ 1.5
Hint
First use an indicator that gives a wide range to find the rough position. Then add indicators whose transition ranges lie near it and find the overlap.
Q3—
Hint
Write down the range each indicator gives. Then find the overlap that satisfies both.
Q4 · Fill in the blankFill in the blank: most indicators change color over a transition range about [＿＿＿＿] wide. (Write a number and a unit, e.g. ○ pH units)
Hint
Methyl red 4.5–6.0, phenolphthalein 8.2–10.0 — subtract to get each width yourself (round to a whole number).
Q5 · Apply (advanced, not required for completion)When a weak acid is titrated with a strong base, the pH at the equivalence point is about 8.8. Which indicator is better for detecting this equivalence point: methyl red or phenolphthalein? (type the name)
Reflect Reflect — metacognition · completion
My prediction result: Not predicted yet
Explain in one or two sentences why the statement "if phenolphthalein is colorless, the solution is acidic" is wrong. Use the words "transition range":
Apply Use the idea in a new context (not required for completion)
Transfer task: nature also has indicators, such as red cabbage juice or hydrangea flowers (their color depends on soil pH).
Universal indicator paper is made from a mixture of several indicators, not just one. Explain why, using today's ideas of "transition range" and "overlapping ranges".
Next up What to study next
Ka and percent ionization of weak acids (an indicator is also a weak acid) → titration curves: choosing an indicator that matches the pH at the equivalence point.
Self-study Self-check key
The worksheet answers open here after you finish every step (predict, observe, check, reflect). Try to solve it yourself first.
COMPLETE · Completed — all checks passed.
Worksheet — Transition ranges of acid–base indicators (black and white)
Name ________ Class ________ Date ________
1. Predict — A solution with phenolphthalein is colorless. What can you say for certain? (it is acidic / its pH is below about 8.2 / it is neutral)
2. Observe — Use the chart above (outlined bar = transition range; words beside the bar = acid-form color / base-form color) to complete the table.
Indicator
Acid-form color
Transition range (pH)
Base-form color
Methyl red
____ – ____
Bromthymol blue
____ – ____
Phenolphthalein
____ – ____
3. Infer — Methyl red is yellow and phenolphthalein is colorless. What is the possible pH range? [ ____ ] ≤ pH ≤ [ ____ ]
4. Fill in the blank — Most indicators change color over a transition range about [ ________ ] wide (write a number and a unit).
5. Conclusion — Use the transition range to explain why "colorless phenolphthalein = acidic" is wrong.
Answer key (separate teacher page)
1. Its pH is below about 8.2. 2. Methyl red: red / 4.5–6.0 / yellow · Bromthymol blue: yellow / 6.0–7.6 / blue · Phenolphthalein: colorless / 8.2–10.0 / pink.
3. 6.0 ≤ pH ≤ 8.2. 4. 2 pH units. 5. Colorless means the pH is below the transition range (about 8.2). Neutral and weakly basic solutions with pH 7–8.2 are also colorless.
Teacher notes — open the lesson plan
Lesson plan · 5E phase and time
Explore and Explain, 15 min. Use before a lab (indicator colors, neutralization) as preparation, or after the lab as a summary.
Demonstration in 3 steps
① Add only phenolphthalein and move pH 2 → 7 → 7.8 → 9 → 11. Stress that it is still colorless at 7.8.
② Add methyl red + bromthymol blue + phenolphthalein together and compare pH 5, 7 and 9.
③ Switch to unknown sample mode. Let students discuss the order of indicators, then use "Reveal unknown pH" in the teacher bar.
Three key questions
"Which pH values does the observation 'colorless' rule out, and which does it not rule out?"
"Why does each indicator change color at a different pH, not at pH 7?" (their pKa values differ)
"Why does pH paper contain a mixture of several indicators?"
Common misconceptions
MC-ind-01: "colorless phenolphthalein = acidic" → colorless means pH < about 8.2, which includes neutral and weakly basic solutions.
MC-ind-02: "all indicators change at pH 7" → methyl violet 0–1.6, alizarin yellow R 10.1–12.0, and so on: each is different.
MC-ind-03: "one indicator gives the exact pH" → one indicator gives only a range (use a pH meter for precise measurement).
Note on photographs: published photos of salt solutions with indicators may show colors that do not match the range definitions (for example, solutions inside a transition range shown as pure red or yellow). Photo colors are approximate; this simulation follows the range definitions.
Answer key (hidden in student mode)
Entry check: a weak-acid dye whose acid and base forms have different colors. Prediction: pH is lower than about 8.2. Q1: about 6.0–7.6.
Q2: Basic, unknown pH 5.2 (methyl red alone gives width 1.5) / Standard 9.0 (phenolphthalein + thymol blue → 8.2–9.6, width 1.4) / Advanced 7.8 (bromthymol blue blue [7.6, 14] ∩ thymol blue yellow [2.8, 8.0] ∩ phenolphthalein colorless [0, 8.2] → 7.6–8.0, width 0.4, at least 3 indicators). An earlier advanced version (4.45, methyl orange + methyl red → width 0.1) was dropped: it relied on an artificial gap between model limits and gave false precision.
Q3: Basic 8.2 / Standard 6.0 / Advanced 0.6. Q4: 2 pH units ("2 pH", "2 units" also accepted). Q5: phenolphthalein (8.2–10.0 includes 8.8).
Data sources · verification
Indicator
Value used (pH)
Source
Methyl violet
0.0–1.6 yellow→violet
standard literature value
Thymol blue
1.2–2.8 red→yellow, 8.0–9.6 yellow→blue
standard literature value
Methyl orange
3.1–4.4 red→yellow
standard literature value
Methyl red
4.5–6.0 red→yellow
common teaching value (literature about 4.4–6.2; small difference)
Bromthymol blue
6.0–7.6 yellow→blue
standard literature value
Phenolphthalein
8.2–10.0 colorless→pink
standard literature value (some sources give 8.0–10.0)
Alizarin yellow R
10.1–12.0 yellow→red
standard literature value
Decimal limits follow the standard values of the acid–base indicator table in the CRC Handbook of Chemistry and Physics. They agree within about ±0.3 with indicator range charts read from images. The lower limit of thymol blue and the upper limit of methyl violet differ by 0.2–0.3 between sources; the literature values were used.
University general chemistry · acids and basesEngine E3 state/diagramVerified: checked against standard literature values (2026-10-01)