Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

2.2.1Inorganic Chemistry: Part 1

Indicators — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions

5 min read 7 parts Lesson 8 of 15

📚 Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 5Two-colour indicators

IndicatorColour in acidColour in alkali
LitmusRedBlue
PhenolphthaleinColourlessPink
Methyl orangeRedYellow

Part 2 of 5Choosing an indicator

Litmus
  • Litmus Extracted from lichens, litmus is best used as paper (red or blue) for dipping into solutions or testing gases.
  • It changes colour gradually through purple in neutral solution, so it is not suitable for titrations.
Synthetic indicators
  • Synthetic indicators Phenolphthalein and methyl orange are organic compounds that change colour sharply over a narrow pH range.
  • This sharp change makes them ideal for showing the end-point of a titration.
Universal indicator
  • Universal indicator A mixture of dyes that gives a range of colours across the pH scale.
  • It gives only an approximate pH, so it is not suitable for titration.

Part 3 of 5The pH scale

1pH 0 to 3
  • pH 0 to 3 Strongly acidic; values below 1 are extremely acidic.
2pH 4 to 6
  • pH 4 to 6 Weakly acidic.
3pH 7
  • pH 7 Neutral: neither acidic nor alkaline, as in pure water.
4pH 8 to 10
  • pH 8 to 10 Weakly alkaline.
5pH 11 to 14
  • pH 11 to 14 Strongly alkaline.

Part 4 of 5Why indicators change colour

Indicator behaviour
Weak acids and bases
  • Weak acids and bases Most indicators are themselves weak acids or bases whose molecules and ions have different colours.
  • Adding acid or alkali shifts the balance between the two forms, so the observed colour changes.
Narrow range
  • Narrow range A synthetic indicator switches colour over only about two pH units, which is why it marks the end-point of a titration sharply.
Natural sources
  • Natural sources Plants such as red cabbage and many flowers contain pigments that act as indicators,
    • but their colour changes are usually gradual and less precise.
Practical use
  • Practical use Phenolphthalein is used when titrating a strong alkali against a strong acid,
    • while methyl orange suits titrations of acids with weaker alkalis such as sodium carbonate.

Part 5 of 5Using universal indicator

1
Add a few drops of universal indicator solution to a small sample of the unknown liquid.
2
Swirl gently and note the colour that develops.
3
Compare the colour with the manufacturer's chart to read off an approximate pH.
4
Decide whether the solution is acidic (pH below 7), neutral (7) or alkaline (above 7), and how strongly.

Key terms

TermMeaning
IndicatorA substance that changes colour depending on whether a solution is acidic or alkaline.
pHA scale from 0 to 14 that shows how acidic or alkaline a solution is.
End-pointThe moment in a titration when the indicator changes colour, showing neutralisation.
NeutralHaving a pH of exactly 7.
AlkaliA soluble base that forms hydroxide ions in water and has a pH above 7.

Avoid these mistakes

1
Choosing universal indicator for a titration. It changes colour gradually, so the end-point cannot be seen clearly.
2
Mixing up the colours of methyl orange: it is red in acid and yellow in alkali.
3
Stating that litmus turns purple in alkali. Purple is the transition colour in neutral solution; alkali turns it blue.
4
Assuming a lower pH is more alkaline. The lower the pH, the more acidic the solution.

Last updated: