lesson plan 10 - abc.net.au

Lesson Plan 16 – |

© 2007 | ABC Science Online

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. Duration: 60 - 80 minutes

Year level: Lower to upper primary*

Topics: Natural and process materials, Energy and Change

Preparation: 10 to 20 minutes

Extensions: Investigate the effect of carbon dioxide gas on water using pH Indicators, Investigate antioxidants (anthocyanin is a powerful antioxidant)

Overview

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

Small groups 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

Whole class Discuss the activity (15 – 20 min) Teacher Demo 2: Test pH of soluble aspirin (see Teacher Notes)

Beautiful Results using

Red Cabbage pH Indicator

Materials and equipment

These quantities are for 6 separate groups

Total Quantity Description

Making pH Indicator (sufficient for whole class and demonstrations)

½ 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)

¼ cup Chemical 1 – Tap Water

¼ cup Chemical 2 – White vinegar

1 tsp Chemical 3 – Baking Soda (dissolved in ¼ cup tap water)

¼ cup Chemical 4 – Lemonade (375ml can sufficient for whole class)

1 tsp Chemical 5 – Laundry powder (any brand in ¼ cup water)

¼ 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.



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)



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



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.

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

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

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

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

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.

Each group will only need a tiny amount of the final solution so this is not critical – less than ¼ of a cup.

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



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.

TEACHER DEMO 2:

8. Less than ¼ cup for each group is more than sufficient pH Indicator solution to perform all the chemical tests.

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 ¼ cup 2. Vinegar ¼ cup undiluted 3. Baking Soda 1 tsp in ¼ cup water (stir until clear) 4. Lemonade ¼ cup undiluted 5. Laundry Powder 1 tsp in ¼ cup water (stir until mostly dissolved) 6. Lemon Juice 1/8 cup undiluted 7. Tartaric Acid 1 tsp dissolved in ¼ cup water

Bi-Carb Soda

Laundry Powder

White Vinegar

Tartaric Acid Lemon Juice

Lemonade



SMALL GROUP ACTIVITY:

Preparing Straw Test Tubes and Mini Lab

Each group will require: 1 Plastic plate 4 Straws (cut in half to make 8 micro test tubes) 1 Blob of poster wall putty

1. Cut the four straws neatly in half.

2. Place eight small blobs of poster putty evenly spaced on

the plastic plate. Push halved straws firmly into the putty.

3. Completed Mini-Lab with straw test tubes ready for experiments.

Testing pH of Household Chemicals Rotate the chemicals to be tested around the groups – the pipette and stirrers should remain with their solutions to avoid mixing/contamination.

1. Use a pipette or eye-dropper to add pH Indicator solution

to each test tube so they are all roughly one third full.

2. Add tap water to the first test tube so it is roughly two thirds full (do not overfill so solution can be ‘stirred’ with the shish-kebab stick stirrer if necessary).

3. Gently ‘stir’ the solution by inserting the skewer into the straw several times – students record their observations and classify the chemical on their worksheets.



Testing pH of Household Chemicals … /continued

4. Add each test chemical in succession, stir with shish-kebab skewer, record resulting colour change and classify the chemical.

5. Continue until each chemical has been tested and classified. Cleaning up is easy and simple and materials can be used again if washed in warm soapy water.

Results

The following results will be obtained – the concentration of pH indicator will affect the colour intensities but not the hue (colour). Chemical Colour Change Acid/Base Tap Water No noticeable colour change pH neutral Baking Soda Blue to blue-green Base Vinegar Red to purplish-red Acid Tartaric Acid Bright red Acid Laundry Powder Yellow-green Base Lemonade Purple Acid Lemon Juice Bright Red Acid

Bak

ing

Sod

a (b

lue

or b

lu-g

reen

)

Vin

egar

(re

d-pu

rple

)

Tarta

ric A

cid

(brig

ht re

d)

Laun

dry

Pow

der

(yel

low

-gre

en)

Lem

onad

e (p

urpl

e)

Lem

on J

uice

(b

right

red)

Tap

Wat

er

(vio

let -

no

chan

ge)



TEACHER DEMO 2 - CONCLUSION: Testing Soluble Aspirin

During the concluding discussion, use a large glass to observe the change in pH of water as a soluble aspirin dissolves. Use a large, tall glass so the whole class can see the change.

The solution slowly turns pink indicating that the pH is decreasing (ie the solution is becoming acidic).

Acids, Bases and pH Indicators

The pH scale ranges from 1 to 14. A chemical with a pH of 7 is neutral – it is neither and acid or a base. The nomenclature pH means “the potential of hydrogen” and refers to the ability of a chemical to donate or accept hydrogen ions to other chemicals. Acids donate hydrogen ions while bases can accept them. Acids are chemicals with a pH less than 7. The word “acid” comes from the Latin acidus meaning “sour” because acids generally have a sour taste. Some acids such as sulfuring and nitric acid are particularly strong and can severely burn skin while other acids are much weaker and safer. Most of the foods humans eat are acidic. A base is a chemical with a pH greater than 7. Bases generally have a bitter taste and a slimy or soapy feel to the skin (this sensation is very noticeable in laundry powders). Bases react violently with acids – even weak acids and bases such as acetic acid (vinegar) and sodium bicarbonate (baking soda) fizz violently to release carbon dioxide gas yet both are safe to consume. Acids and bases react with each other to produce a salt and water (there are many types of salts of which sodium chloride which is common table salt is just one). pH indicators (acid-base indicators) are chemicals which change colour in the presence of an acid or a base. The change is reversible. Adding an acid or base to water decreases or increases the solution’s pH and the indicator colour will change accordingly.





pH of some common substances pH Substance 1.0 Battery Acid (sulfuric acid) 1.8-2.0 limes 2.2-2.4 lemon juice 2.2 vinegar (acetic acid) 2.9-3.3 apple juice, cola 3.7 orange juice 4.0-4.5 tomatoes 5.6 unpolluted rain 5.8-6.4 peas 6.4 cow's milk 6.5-7.5 human saliva 7.0 distilled water 7.3-7.5 human blood 8.3 baking soda 9.2 borax 11.0 laundry ammonia 12.0 lime water 14.0

Red Cabbage Indicator - Anthocyanin

The chemicals responsible for the red colour in red cabbage and many other vegetables, fruits and flower petals belong to the anthocyanin family. Over 300 kinds of anthocyanin have been discovered so far. They are also powerful antioxidants which have been shown to be very beneficial to human health. The following links provide further reading about anthocyanin. Food Pigments Stop Cancer in its Tracks (20/08/07) Sydney Morning Herald Article www.smh.com.au/news/national/food-pigments-stop-cancer-in-its-tracts/2007/08/20/1187462176713.html Red Leaves (23/02/05) ABC TV – Catalyst (Transcript) www.abc.net.au/catalyst/stories/s1310369.htm Red Leaves (May 2003) Tropical Topics No. 77 Page 6 (Queensland Environmental Protection Authority) www.epa.qld.gov.au/register/p00820ak.pdf Anthocyanin in the Rainforest (05/09/92) Tropical Topics - Queensland Environmental Protection Authority www.epa.qld.gov.au/register/p00820ar.pdf

In this activity, you will extract amazing natural chemicals from red cabbage. These chemicals change colour when added to acids or bases.

You will build a MiniLab complete with Micro Test Tubes made from straws. Add your cabbage acid-base indicator to test various chemicals are an acid or base.

Objectives

To build a MiniLab with Micro Test Tubes To extract the natural acid-base indicators from red cabbage leaves. To use the acid-base indicators and MiniLab to test whether chemicals are acids or bases. To work cooperatively so every group member contributes to the activity To conserve water by using the minimum amount required To conserve materials and recycle them where possible Making the MiniLab Materials required

Plastic Plate Adhesive poster putty 4 Clear Straws Scissors

Procedure Work cooperatively to build your MiniLab and try to make sure everyone has a turn at part of the procedure. The equipment manager collects the materials required from the science store.

1. Cut the four straws neatly in half. You should now have eight shorter straws of equal length. These will become your MiniLab’s Micro Test Tubes.

2. Place seven small blobs of poster putty evenly spaced on the plastic plate (it doesn’t matter if your plate is round or square).

3. Push seven of the halved straws firmly into the putty.

This is how your completed MiniLab should look.


Preparing the Acid-Base Indicator

Materials required

Red Cabbage 1 Clear Plastic Cup 1 Tissue Small funnel

Procedure

The equipment manager collects the red cabbage prepared by the teacher. Testing the Chemicals

Procedure

Collect one of the chemicals to test. Once you have finished testing, pass the chemical onto another group and collect the next chemical to be tested.

1. Line the funnel with a facial tissue. Carefully pour the water from the red cabbage into funnel. Try not to spill any of cabbage.

2. Carefully put the leftover cabbage and tissue back into the container. Compost the leftover cabbage and tissue if possible.

3. Use the eye-dropper to add Acid-Base Indicator solution to each test tube so they are all roughly one third full. You need to leave room for the chemicals you will test.

1. Add tap water to the first test tube so it is roughly two thirds full.

This test tube will act as your reference colour for a neutral

chemical (one that is neither an acid nor a base).

2. Gently ‘stir’ the water into the acid-base indicator solution with the shish-kebab skewer.

3. Repeat steps 1 and 2 for each chemical to be tested. Record your observations in your science journal.

Red Cabbage Indicator Colour Chart

pH less than 7 = Acid pH more than 7 = Base pH

2 4 6 8 10 12

Colour

Red Purple Violet Blue Blu-Grn Grn-Yel

Note: This colour chart is a guide only as colours may vary depending on your printer

Table 1. Acid Indicator Tests Results & Analysis

Chemical Indicator Colour Acid or Base

1. Tap Water

2. White Vinegar

3. Baking Soda (Sodium Bicarbonate)

4. Lemonade

5. Laundry Powder

6. Lemon Juice

7. Tartaric Acid

Free Experimentation

When you have completed all your tests, try mixing several chemicals to see if you can change the indicator colour from purple to pink, then to blue and back to pink again. Using very small quantities of acids and bases is the best way to achieve this result. Apart from the colour change, you will also notice that some chemicals produce bubbles of gas when they react.


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