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Indicators and pH

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Indicators and pH

Summary: The pH scale (0-14) measures acidity and alkalinity: pH < 7 is acidic, pH 7 is neutral, pH > 7 is alkaline. Universal indicator shows the full pH range. Litmus, methyl orange, and phenolphthalein have specific colour changes used in titrations and qualitative analysis. Tags: igcse chemistry acids-bases Created: 2026-07-14 Last Updated: 2026-07-16

Learning Objectives

By the end of this topic, you should be able to:

  • Define the pH scale (0-14) and relate pH to acidity and alkalinity
  • Describe the colour changes of litmus, methyl orange, phenolphthalein, thymolphthalein, and universal indicator in acidic, neutral, and alkaline solutions
  • Explain why universal indicator is more useful than litmus for determining the strength of an acid or alkali
  • Select an appropriate indicator for a given titration
  • Explain the difference between a strong acid and a weak acid in terms of pH

Content

1. The pH Scale

The pH scale is a measure of the acidity or alkalinity of an aqueous solution:

pH 0 ← increasing acidity | NEUTRAL | increasing alkalinity → pH 14
pH RangeClassification[H+] relative to [OH-]
0-2Strongly acidic[H+] > [OH-]
3-6Weakly acidic[H+] slightly > [OH-]
7Neutral[H+] = [OH-]
8-11Weakly alkaline[H+] < [OH-]
12-14Strongly alkaline[H+] < [OH-]

Key concept: The pH scale is logarithmic — each decrease of 1 pH unit represents a 10-fold increase in hydrogen ion concentration [H+].

  • pH 3 has 10 times more H+ ions than pH 4
  • pH 3 has 100 times more H+ ions than pH 5
  • pH 3 has 1000 times more H+ ions than pH 6

2. pH and Acid/Alkali Strength

Strong vs Weak acids (IGCSE level):

  • Strong acid: completely ionises in water (e.g., HCl → H+ + Cl-; H2SO4 → 2H+ + SO4 2-; HNO3 → H+ + NO3-)
  • Weak acid: partially ionises in water (e.g., CH3COOH ⇌ CH3COO- + H+; H2CO3)

At the same concentration (e.g., 0.1 mol/dm3):

  • A strong acid has a lower pH (more H+ ions in solution) than a weak acid
  • HCl (0.1 mol/dm3) has pH ≈ 1; CH3COOH (0.1 mol/dm3) has pH ≈ 3

Strong vs Weak vs Concentrated vs Dilute:

  • Strength (strong/weak) refers to degree of ionisation — NOT concentration
  • Concentration (concentrated/dilute) refers to how much acid/alkali is dissolved in a given volume of water
  • You can have a dilute strong acid (e.g., 0.001 mol/dm3 HCl, pH ≈ 3) or a concentrated weak acid

3. Universal Indicator

Universal indicator is a mixture of several indicators that shows a range of colours across the pH scale. It allows you to estimate the pH of a solution to within 1 pH unit.

Colour chart:

pHColourDescription
0-1RedStrong acid
2-3OrangeAcid
4-5YellowWeak acid
6Yellow-greenVery weak acid
7GreenNeutral
8-9Blue-greenVery weak alkali
10-11BlueWeak alkali
12-13Violet/PurpleStrong alkali
14Dark purpleStrong alkali

Advantage of universal indicator over litmus: Universal indicator tells you how strongly acidic or alkaline a solution is (approximate pH), whereas litmus only tells you whether it is acidic or alkaline (red or blue).

4. Specific Indicators for Titrations

Different indicators change colour at different pH values. Choosing the correct indicator for a titration is important:

IndicatorColour in AcidColour in AlkalipH Range of Colour ChangeSuitable for…
LitmusRedBlue~5-8Quick acid/alkali test (not titrations)
Methyl orangeRedYellow3.1-4.4Strong acid + weak base OR strong acid + strong base
PhenolphthaleinColourlessPink8.3-10.0Strong base + weak acid OR strong base + strong acid
ThymolphthaleinColourlessBlue9.3-10.5Strong base + weak acid
Screened methyl orangeRed (acid)Green (alkali)—Alternative to methyl orange

Key rule for choosing a titration indicator:

  • Strong acid + strong base: either methyl orange or phenolphthalein (sharp pH change at equivalence point, ~3-11)
  • Strong acid + weak base: methyl orange (equivalence point is acidic, pH ~5-6)
  • Strong base + weak acid: phenolphthalein (equivalence point is alkaline, pH ~8-9)

5. Soil pH and its Importance

Soil pH affects plant growth because:

  • Most plants grow best in soil of pH 6.0-7.5 (slightly acidic to neutral)
  • At extreme pH values, certain nutrients become insoluble and unavailable to plants
  • Acidic soil (pH too low): add lime (calcium hydroxide / calcium oxide / calcium carbonate) to neutralise
  • Alkaline soil (pH too high): add compost, manure, or aluminium sulfate to lower pH

6. Using a pH Meter / pH Probe

A pH meter (or pH probe connected to a datalogger) gives a precise, numerical pH reading. Advantages over indicators:

  • More accurate (e.g., pH 5.63)
  • Works with coloured solutions (where indicator colour change would be hard to see)
  • Can be used for continuous monitoring and computer logging
  • No subjective colour judgement needed

7. Indicators from Natural Sources

Some plants contain pigments that act as natural indicators:

  • Red cabbage: contains anthocyanin — red in acid, purple in neutral, green/yellow in alkali
  • Litmus: originally extracted from lichens
  • Hydrangea flowers: blue in acidic soil, pink in alkaline soil

Worked Examples (Exam-Style)

Example 1: Choosing an Indicator

Question: A student titrates hydrochloric acid (strong acid) against sodium hydroxide (strong base). Suggest a suitable indicator and state the colour change observed at the end point. [2 marks]

Solution (model answer):

  • Phenolphthalein (or methyl orange) ✓
  • Phenolphthalein: colourless in acid → pink in alkali (pink to colourless at end point). OR Methyl orange: red in acid → yellow in alkali (yellow to red at end point) ✓

Example 2: Strong vs Weak Acid

Question: Equal volumes of 0.1 mol/dm3 hydrochloric acid and 0.1 mol/dm3 ethanoic acid are tested with universal indicator. Explain why the two solutions give different pH values. [3 marks]

Solution (model answer):

  • Hydrochloric acid is a strong acid; it completely ionises/dissociates in water to give H+ ions ✓
  • Ethanoic acid is a weak acid; it only partially ionises/dissociates in water ✓
  • At the same concentration, HCl produces a higher concentration of H+ ions, giving a lower pH ✓

Example 3: pH and Concentration

Question: Solution A has pH 2. Solution B has pH 5. How many times greater is the H+ ion concentration in solution A compared to solution B? [1 mark]

Solution (model answer):

  • pH difference = 3 → 10^3 = 1000 times more H+ ions in solution A ✓

Practice Questions

  1. State the colour of universal indicator in: (a) pH 3, (b) pH 7, (c) pH 12. [3 marks]

  2. Explain why universal indicator is more useful than litmus for comparing the acidity of two different acids. [2 marks]

  3. A titration is carried out between ethanoic acid (weak acid) and sodium hydroxide (strong base). Suggest a suitable indicator and explain your choice. [2 marks]

  4. A sample of 0.1 mol/dm3 HCl has pH 1. A sample of 0.1 mol/dm3 CH3COOH has pH 3. Explain what this tells you about the strength of the two acids. [3 marks]

  5. A farmer finds that his soil has pH 5. Name a substance he could add to the soil to raise the pH and explain why this works. [2 marks]

Key Facts to Memorise

  • pH scale: 0-14; below 7 = acidic, 7 = neutral, above 7 = alkaline
  • Each pH unit: 10x change in [H+]
  • Universal indicator colours: red (acid) → orange → yellow → green (neutral) → blue → violet/purple (alkali)
  • Litmus: red in acid, blue in alkali
  • Methyl orange: red in acid, yellow in alkali (end point: yellow → red/orange)
  • Phenolphthalein: colourless in acid, pink in alkali (end point: pink → colourless)
  • Thymolphthalein: colourless in acid, blue in alkali
  • Strong acid: fully ionised in water (HCl, H2SO4, HNO3)
  • Weak acid: partially ionised in water (CH3COOH, H2CO3)
  • Strong vs concentrated: different concepts — don’t confuse them

Common Misconceptions

Students often think…But the correct understanding is…
”All acids have a low pH like 1 or 2”Weak acids have much higher pH values e.g., 0.1 mol/dm3 ethanoic acid has pH ~3
”A concentrated weak acid has a lower pH than a dilute strong acid”It depends on the specific concentrations and degree of ionisation — you cannot generalise
”Methyl orange turns orange at neutral”Methyl orange is yellow at neutral — it changes from red to yellow over pH 3.1-4.4
”Indicators tell you the exact pH”Only universal indicator gives approximate pH; litmus/methyl orange/phenolphthalein only tell you whether things are acidic or alkaline
”Phenolphthalein is pink in acid”Phenolphthalein is colourless in acid and only turns pink above pH ~8.3
”pH 7 water is the only neutral substance”Any solution where [H+] = [OH-] is neutral. Some salts dissolve to give neutral solutions, others are acidic or alkaline

Key Concepts from Past Papers

Definitions You MUST Know (Exact Mark Scheme Wording)

  • pH: a measure of the acidity or alkalinity of a solution on a scale from 0 to 14
  • Indicator: a substance that changes colour depending on the pH of the solution
  • Strong acid: an acid that fully/completely ionises/dissociates in aqueous solution
  • Weak acid: an acid that partially ionises/dissociates in aqueous solution

Recurring Mark Scheme Answers

  • “Universal indicator gives a range of colours across the pH scale / litmus only shows red or blue”
  • “Phenolphthalein changes from colourless to pink / pink to colourless at the end point”
  • “Strong acid has lower pH because it has a higher concentration of H+ ions”
  • “Adding lime (calcium hydroxide/calcium oxide) to acidic soil neutralises it”
  • “Salt + carbon dioxide + water” (for carbonate + acid reactions)

Common Question Types

  • Short recall (state/give/name): “State the colour of methyl orange in an alkaline solution”
  • Extended writing (describe/explain/suggest): “Explain the difference between a strong acid and a weak acid”
  • Multiple choice / tick-box: “Which indicator is suitable for…”
  • Table completion: “Complete the table showing indicator colours”
  • Calculation/interpretation: “Solution A has pH 2; Solution B has pH 5. Compare the H+ concentrations”

Exam Tips

  • This topic appears in 38 papers in the database
  • Total Q+A entries: 90
  • Average marks per question: 1.1
  • Always spell phenolphthalein correctly — it costs no marks if wrong but looks more professional
  • Don’t confuse the colour changes: methyl orange = red → yellow, phenolphthalein = colourless → pink
  • For “strong vs weak acid” questions, the key phrase is “completely ionises” vs “partially ionises”

Keywords from Past Papers

acid, add, indicator, colour, aqueous, precipitate, carbon, dioxide, white, sodium, hydroxide, higher, chart, nitrogen, limewater

Sources

  • OpenStax Chemistry 2e — [Chapter 14: Acid-Base Equilibria (pH and Indicators)], Rice University (free, CC BY 4.0)
  • BBC Bitesize GCSE Chemistry — [pH Scale and Indicators], BBC (free educational resource)
  • Cambridge IGCSE Chemistry 0620 — Syllabus Section 7: Acids, Bases and Salts (Indicators and pH), Cambridge Assessment International Education
  • CK-12 Chemistry for High School — [Chapter 21: pH and Indicators], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

  • 0620/31 May/June 2015: Q11(c)(i) (0m), Q66(a)(i) (0m), Q66(a)(ii) (0m) (+1 more)
  • 0620/32 Feb/March 2015: Q66(e)(ii) (2m), Q66(b)(ii) (1m), Q77(d)(i) (0m)
  • 0620/32 Feb/March 2017: Q22(f)(ii) (1m), Q44(d)(i) (1m), Q55(f)(i) (1m) (+1 more)
  • 0620/32 Feb/March 2018: Q88(b)(ii) (3m), Q22(a)(iv) (2m), Q77(b)(i) (1m) (+1 more)
  • 0620/32 Feb/March 2019: Q44(b)(ii) (1m)
  • 0620/32 Feb/March 2020: Q55(d)(iii) (2m), Q66(a)(ii) (1m), Q55(d)(iii) (2m)
  • 0620/32 Feb/March 2021: Q66(a)(ii) (1m)
  • 0620/32 Feb/March 2022: Q88(d)(ii) (2m), Q44(a)(ii) (0m)
  • 0620/32 Feb/March 2023: Q33(b)(ii) (1m), Q88(c)(iii) (1m)
  • 0620/32 May/June 2018: Q33(b)(i) (1m), Q33(b)(ii) (1m)
  • 0620/32 May/June 2020: Q88(e)(i) (1m)
  • 0620/33 May/June 2016: Q88(c)(ii) (1m), Q88(c)(iii) (0m)

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