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:
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)
Lesson Plan 16 – | Page 1
© 2007 | ABC Science Online
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.
Lesson Plan 16 – | Page 2
© 2007 | ABC Science Online
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
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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)
Lesson Plan 16 – | Page 3
© 2007 | ABC Science Online
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)
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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
Lesson Plan 16 – | Page 4
© 2007 | ABC Science Online
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 ¼ 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
Lesson Plan 16 – | Page 5
© 2007 | ABC Science Online
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 ¼ 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
Tartaric Acid
Lemon
Juice
¼ cup
¼ cup undiluted
1 tsp in ¼ cup water (stir until clear)
¼ cup undiluted
1 tsp in ¼ cup water (stir until mostly dissolved)
1
/8 cup undiluted
1 tsp dissolved in ¼ cup water
Lemonade
White
Vinegar
Laundry
Powder
Bi-Carb
Soda
Lesson Plan 16 – | Page 6
© 2007 | ABC Science Online
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 shishkebab 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.
Lesson Plan 16 – | Page 7
© 2007 | ABC Science Online
Testing pH of Household Chemicals … /continued
4. Add each test chemical in succession, stir with shishkebab 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).
(violet - no change)
Acid/Base
pH neutral
Base
Acid
Acid
Base
Acid
Acid
Tap Water
(blue or blu-green)
Baking Soda
(red-purple)
Vinegar
(bright red)
Tartaric Acid
(yellow-green)
Colour Change
No noticeable colour change
Blue to blue-green
Red to purplish-red
Bright red
Yellow-green
Purple
Bright Red
Laundry Powder
(purple)
Lemonade
(bright red)
Lemon Juice
Chemical
Tap Water
Baking Soda
Vinegar
Tartaric Acid
Laundry Powder
Lemonade
Lemon Juice
Lesson Plan 16 – | Page 8
© 2007 | ABC Science Online
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.
Lesson Plan 16 – | Page 9
© 2007 | ABC Science Online
pH of some common substances
pH
1.0
1.8-2.0
2.2-2.4
2.2
2.9-3.3
3.7
4.0-4.5
5.6
5.8-6.4
6.4
6.5-7.5
7.0
7.3-7.5
8.3
9.2
11.0
12.0
14.0
Substance
Battery Acid (sulfuric acid)
limes
lemon juice
vinegar (acetic acid)
apple juice, cola
orange juice
tomatoes
unpolluted rain
peas
cow's milk
human saliva
distilled water
human blood
baking soda
borax
laundry ammonia
lime water
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-itstracts/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
Lesson Plan 16 – | Page 10
© 2007 | ABC Science Online
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
4 Clear Straws
Adhesive poster putty
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.
© 2007 | ABC Science Online
Preparing the Acid-Base Indicator
Materials required
Red Cabbage
1 Tissue
1 Clear Plastic Cup
Small funnel
Procedure
The equipment manager collects the red cabbage prepared by the teacher.
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.
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. 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
Colour
pH less than 7 = Acid
pH more than 7 = Base
2
4
6
8
10
12
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.
© 2007 | ABC Science Online