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
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
| Substance | Litmus colour | Type |
|---|---|---|
| Hydrochloric acid | Red | Acid |
| Lemon juice | Red | Acid |
| Water | Purple | Neutral |
| Sodium hydroxide | Blue | Alkali |
| Calcium hydroxide | Blue | Alkali |
- 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 substance | Colour | pH |
|---|---|---|
| Strong acid | Red | 1 |
| Weak acid | Yellow | 4 |
| Neutral | Green | 7 |
| Weak alkali | Blue | 10 |
| Strong alkali | Purple | 13 |
- 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
Marked practiceQuizzes for this lesson
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