Lesson Plan 10 - Australian Broadcasting Corporation

Lesson Plan 16

Cool Chemistry - DIY pH Indicator

Brief description

Red cabbage juice is a natural pH indicator making this classic activity one of the most popular chemistry experiments for children. The indicator changes from purple to bright pink in acids to blue or yellow-green in bases. Preparation time and consumables have been greatly reduced in this lesson plan and the whole activity can be conducted at virtually no cost. The pH indicator is extracted as a teacher demonstration while students build their own MiniLabs using clear straws as test tubes to dramatically reduce the amount of indicator and chemicals used. They use the pH indicator to classify a variety of safe household chemicals including vinegar, laundry powder and lemonade.

Beautiful Results using Red Cabbage pH Indicator

Duration: Year level: Topics: Preparation: Extensions:

60 - 80 minutes Lower to upper primary* Natural and process materials, Energy and Change 10 to 20 minutes Investigate the effect of carbon dioxide gas on water using pH Indicators, Investigate antioxidants (anthocyanin is a powerful antioxidant)

Overview

Whole class

Small groups Whole class

Discuss acids and bases and pH (Teacher Notes P 5),

(15 ? 20 min)

Perform Teacher Demo 1 ? Extracting Red Cabbage Acid-Base Indicator (see Teacher Notes)

Perform Teacher Demo 2 ? Preparing Test Chemicals (Teacher Notes P 6)

Distribute worksheets, discuss small group activities and safety precautions

Allocate groups and jobs

Construct MiniLabs and Classify Chemicals

(30 ? 40 min)

Students follow worksheet instructions to construct MiniLabs

Students filter the cooled pH indicator solutions

extracted in Teacher Demo 1.

Students test household chemicals and record observations

Discuss the activity

(15 ? 20 min)

Teacher Demo 2: Test pH of soluble aspirin (see Teacher Notes)

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Materials and equipment

These quantities are for 6 separate groups

Total Quantity

Description

Making pH Indicator (sufficient for whole class and demonstrations)

?

Red cabbage

1

Chopping board and knife

1

Kettle (to boil water for extracting indicator)

1

Small funnel (for filtering cabbage extract)

6

Takeaway containers (1 per group)

6

Plain facial tissues (1 per Group - for filtering cabbage extract)

14

Clear plastic cups (1 per group, 1 Demo + 7 test chemicals)

8

Plastic pipettes or eye-droppers (1 per test chemical)

7

Shish-kebab sticks (1 per chemical - snap off sharp ends)

? cup

Chemical 1 ? Tap Water

? cup

Chemical 2 ? White vinegar

1 tsp

Chemical 3 ? Baking Soda (dissolved in ? cup tap water)

? cup

Chemical 4 ? Lemonade (375ml can sufficient for whole class)

1 tsp

Chemical 5 ? Laundry powder (any brand in ? cup water)

? cup

Chemical 6 ? Lemon Juice (squeeze bottle type is suitable)

1 tsp

Chemical 7 ? Tartaric Acid (available from supermarket)

1

Soluble aspirin for Teacher Demo (optional)

1

Tall glass for Teacher Demo

Materials for making Min-Labs

24

Clear plastic straws (4 per group)

6

Strips of adhesive poster putty (1 per group)

6

Plastic plates (1 per group)

1

Absorbent table cloth (or towel)

Preparation

Purchase and/or collect red cabbage (half is sufficient), vinegar, baking soda, laundry powder, lemonade, lemon juice, tartaric acid and soluble aspirin from home and/or supermarket. Snap off sharp ends of shish-kebab skewers for safety.

Photocopy sufficient quantity of student worksheets for whole class.

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Objectives

Students' prior knowledge

No prior knowledge is required or assumed for this lesson plan. The objectives listed below are suggestions only and may not be appropriate for every year level.

Science skills

Students will:

? Follow worksheet instructions to construct MiniLabs ? Use funnels and cups to carefully filter the pH indicator solution

from red cabbage leaves ? Use eye-droppers to carefully add pH indicator and test

chemicals to test tubes (clear straws) ? Record their observations on the student worksheet ? Classify each test chemical as an acid or base on their worksheet

Science concepts

? pH is a measure of how acidic or basic a chemical ? The pH scale ranges from 1 to 14 ? A pH of 7 is neutral ? it is neither basic nor acidic ? A pH less than 7 is acidic ? A pH greater than 7 is basic ? pH indicators are chemicals that change colour when added to

an acid or base ? red cabbage juice contains natural pH indicators ? red cabbage pH indicator turns pink or red in acids ? red cabbage pH indicator turns blue to yellow-green in bases ? the pH indicators in red cabbage juice belong to the

anthocyanin family of chemicals

Positive attitudes

Students will

? Develop an understanding of and appreciation for the scientific method

? Work cooperatively with partners/group members and ensure ? Handle all equipment and water carefully and responsibly ? Dispose of waste responsibly (eg pour waste water onto plants

or garden beds instead of down the sink)

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Procedure

Teacher Demonstration ? Extract pH Indicator Whole Class (10?15 min)

? Discuss students' perceptions of chemicals by asking leading questions such as "what are chemicals?" "are all chemicals dangerous? are all chemicals liquids?" "is water a chemical? what about lemonade or vinegar?" "what do you know about acids and bases?" "are all acids dangerous?"

? Display the red cabbage and ask students whether it contains any unusual or interesting chemicals. Explain that it contains many thousands of amazing chemicals which scientists are still learning about including some that change colour when mixed with acids or bases.

? Students who have a pool at home may already be familiar with pH indicators because pool maintenance includes measurement of pH using indicators

? Introduce the lesson ? you are going to demonstrate how to extract the natural, colour changing acid-base indicator from the cabbage. The class will then break into small groups, build their own mini chemistry lab complete with micro test tubes and use some of the red cabbage indicator to test the pH of various household chemicals.

? Follow the procedure on page 5 of the Teacher Notes to extract the pH Indicator Solution ? stop when hot water is added at Step 5 and continue class discussion (the final filtering steps will be performed by students in small groups when the solution has cooled)

? Discuss the procedure for small group activities ? Allocate groups and jobs (allocating job badges will minimise classroom traffic and the

risk of accidents and spills)

Build Mini Labs and Test Chemicals ? Small Groups (15?20 min)

? All group members read the worksheet instructions ? Equipment Manager collects materials required for activity from science store ? Group members cooperate to build the MiniLabs ? Equipment Manager collects cooled cabbage juice for straining ? Each group member has a turn at adding chemicals to test tubes ? All group members record observations and classify chemicals on their worksheets ? Free Experimentation ? students will enjoy mixing several chemicals together to

observe the colour change. Allow up to 15 minutes of free time for experimentation. All the chemicals are safe to mix and can be discarded onto a garden or poured down the sink. ? All group members cooperate to clean up ? Equipment manager returns equipment to science store ? Place red cabbage scraps and tissues in compost bin if possible

Discussion ? Whole Class (10?15 min)

? Discuss the activity and observations ? Scientists and industrial chemists use a variety of pH indicators to measure the pH and

concentrations of solutions very accurately ? Perform Teacher Demo 2: Testing Soluble Aspirin and discuss

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Teacher's notes

TEACHER DEMO 1: Preparing Red Cabbage pH Indicator Solution

There are no critical steps in this procedure. Chopping the cabbage more finely, using hotter water and longer soaking times will all extract more of the desired chemical from the cabbage. Perform this as a demonstration rather than preparing in advance ? students will enjoy seeing the process and will be more amazed if the realise how simple the extraction of the pH indicators from red cabbage is.

1. Half a red cabbage will provide ample pH indicator for the whole class, plus teacher demonstrations

2. Finely chop the cabbage on a large chopping board.

The finer you chop the cabbage, the better but it is not critical.

3. Evenly distribute the chopped cabbage into 6 takeaway containers.

Each group will only need a tiny amount of the final solution so this is not critical ? less than ? of a cup.

4. Pour enough boiling or hot water into each container to just cover the cabbage.

The volumes and water temperature are not critical ? hotter water is better.

5. Stirring will extract more of the pH indicators but is not critical ? stop at this point and continue class discussion.

This step is not critical ? a longer soak time will extract more pH indicator

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6. Line a small funnel with a facial tissue and strain the water from the cabbage into a plastic cup.

NOTE: This step can be completed by students in small groups or by the teacher once the water has cooled sufficiently ? 10 to 15 minutes is sufficient extraction time but you could begin the lesson prior to a break and allow soaking

7. Remove tissue and leftover cabbage and place back in container ? compost leftovers and tissue if possible.

8. Less than ? cup for each group is more than sufficient pH Indicator solution to perform all the chemical tests.

TEACHER DEMO 2: Preparing Chemical Solutions for Testing

Label six plastic and fill as follows. Only one set is required for the whole class and will be rotated through each of the groups:

1. Tap water 2. Vinegar 3. Baking Soda 4. Lemonade 5. Laundry Powder 6. Lemon Juice 7. Tartaric Acid

? cup ? cup undiluted 1 tsp in ? cup water (stir until clear) ? cup undiluted 1 tsp in ? cup water (stir until mostly dissolved) 1/8 cup undiluted 1 tsp dissolved in ? cup water

Tartaric Acid

Lemon Juice

Lemonade

White Vinegar

Laundry Powder

Bi-Carb Soda

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