Unit 10 Common Acids and Alkalis

Topic

Acid-base Indicators from Natural Materials

Curriculum Link

10.1 Common Acids and Alkalis
10.2 Indicators for testing acids and alkalis

Estimated Lesson Time

80 minutes

Introduction

The use of Indicators to determine whether an aqueous solution is acidic or basic is important for students to learn acids & alkalis. In this activity, we start with the meaning of acids & alkalis, then introduce 2-3 common indicators. Finally we introduce the use of Universal Indicator to measure pH.

In practice, these common acid-base indicators are often extracted from plant materials. So we would like the students to practice on hands how they can extract some useful indicators from flower petals, leaves or even roots. Then test them on acidic and alkaline solutions.

Key Question

What is the mechanism of colour change of a pigment in acidic and alkaline solutions? Is this mechanism a reversible reaction?

Learning Objectives

In this activity, the students should be able to

  1. recognise that solutions are classified into neutral, acidic and alkaline;
  2. explore how the colour of a pigment is affected by pH;
  3. acquire planning and designing skills (SP3) in designing an experiment to extract out the pigment from a plant material as an acid-base indicator;
  4. acquire measuring skills (SP1) and experimenting skills (SP4) in extracting pigments by an organic solvent and use it to construct a colour chart;
  5. acquire comparing and classifying skills (SP2) by identifying the pH for acidic and alkaline solutions
  6. acquire interpreting data skills (SP5) in analysing the data collected

Teaching Plan

Task (Time) Brief Description Materials Objectives

Engagement
(10 min)

Teacher shows 2-3 different acid-base indicators and tests them on some common daily solutions to arouse students’ interest. Students’ are asked to predict the acidity of these solutions. Teacher then tests them with the indicators and asks students to complete the table of Q1 on the worksheet.

(1)

Exploration
(30 min)

Students design setup(s) to extract the pigments from 3 different plant materials before carrying out the experiment

  • in a systematic way
  • in compliance with all necessary safety precautions (why using a water bath instead of direct heating)
  • with necessary measure(s) to increase the yield of the pigments (e.g. cutting/grinding the plant materials into small pieces)

Students report their designs with discussion.

Teacher demonstrates how to use 95% ethanol solution to extract the pigment from plant materials by using a hot water bath.

(2), (3) & (4)

Evaluation
(10 min)

Students identify the colours of the 3 pigments extracted on dilute HCl(aq) and NaOH(aq) and record the results in the Table.

(5) & (6)

Elaboration &
Explanation
(10 min)

Students visit the website to propose the composition of universal indicators and attempt to explain the different colour in different pHs.

Students explain how the colour change occurs due to acids (H+) and alkali (OH-) on the reversible ionisation of the indicator.

 Homework Internet search (6)

Further Exploration
(20 min)
  • If time allows, students repeat the experiment with other plant materials.
  • Students can try to mix pigments such that different colour can be seen at different pHs – a home-made universal indicator
 rose, blue berry, rhubarb(大黃) (3) & (4)

*Apparatus and materials required: