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10.6Unit 5: Metals, non-metals, acids and alkalis

Indicators and the pH scale

5 min read 4 parts Lesson 6 of 7 1 quiz

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Lesson summary

10.6 Indicators and the pH scale — CHECKPOINT (CHEMISTRY) notes: Use indicators and the pH scale to tell acids, alkalis and neutral substances apart.Indicator = a substance that turns one colour in acid and another in alkaliPlant indicators — red cabbage, blackcurrant and beetroo…

  • Making an indicator
  • Universal indicator
  • Simple indicators
  • Chemical property
  • Litmus — a common dye, used as litmus paper made by soaking absorbent paper in litmus solution
  • Neutral = neither acid nor alkali

Part 1 of 4Indicators

  • Indicator = a substance that turns one colour in acid and another in alkali
  • Plant indicators — red cabbage, blackcurrant and beetroot can be used
    • Red cabbage turns red in acid, blue in water and yellow in alkali.
  • Making an indicator — crush plant material with a little ethanol and use the coloured liquid; ethanol is flammable

Part 2 of 4Litmus

SubstanceLitmus colourType
Hydrochloric acidRedAcid
Lemon juiceRedAcid
WaterPurpleNeutral
Sodium hydroxideBlueAlkali
Calcium hydroxideBlueAlkali
  • Litmus — a common dye, used as litmus paper made by soaking absorbent paper in litmus solution
  • Neutral neither acid nor alkali

Part 3 of 4Universal indicator

Type of substanceColourpH
Strong acidRed1
Weak acidYellow4
NeutralGreen7
Weak alkaliBlue10
Strong alkaliPurple13
  • Universal indicator — a mixture of indicators that shows how acidic or alkaline a substance is
  • Simple indicators — only show whether a substance is acid or alkali

Part 4 of 4The pH scale

  • pH scalemeasures the strength of acids and alkalis from 0 to 14
  • Below 7acidic; more acidic as the pH gets lower
  • pH 7neutral
  • Above 7alkaline; more alkaline as the pH gets higher
  • Chemical propertythe pH of a substance is one of its chemical properties

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