Kylin Alm
Predict, Observe, Explain
Title and Sources: Red Cabbage Indicator
http://www.abc.net.au/science/surfingscientist/pdf/lesson_plan16.pdf (Red Cabbage Indicator)
http://lrs.ed.uiuc.edu/students/erlinger/water/background/ph.html (Background Information)
Materials:
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Vinegar
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Hydrogen peroxide
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Diet Coke
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Test tubes
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Baking Soda
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Test tube trays
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Glass Cleaner
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Cups/ beakers
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Water
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Pipette
Safety:
Make sure to clean up any spills that occur, the solutions may stain clothes. Do not consume any
of the solutions and wash your hands if you come in contact with any of the solution.
Disposal: all solutions can be rinsed down the drain
Set up:
To make the pH indicator blend two cups of water with one leaf of red cabbage, then strain out
any cabbage chunks from the solution. Pour this purple juice into test tubes. Next, pour hydrogen
peroxide, water, vinegar, baking soda (with water), diet coke and glass cleaner into 6 different beakers,
and place a pipette in each. One at a time use the pipette to squeeze some of the substance being
tested into the test tube with the cabbage juice. Do this for all 6 substances. NOTE: Each test tube
should only be used to test one substance.
What will happen?
The red cabbage juice will change a different colour in response to the different pH values of the
substances that it is mixed with. If an acid is mixed, the indicator will change from a purple to a pink/red
colour, and if it is mixed with a base the solution will change to a blue/green colour depending on how
strong of a base it is. Vinegar will change the solution to a bright pink and diet coke will change it to a
slight redish/pink colour. This tells us that both vinegar and diet coke are acids. Hydrogen peroxide will
Kylin Alm
change the indicator a lighter purple colour, this means that hydrogen peroxide is a weak acid. The
water will keep the solution at a purple colour, this would be our control in the experiment because the
red cabbage is mixed with water to make the solution so mixing water with the indicator shouldn’t
change the colour. This means that water is neutral. Finally, baking soda and glass cleaner change the
indicator to a green and a bluish/green colour indicating that these two substances are bases.
Why it happens?
pH is a measurement of how acidic or how basic a solution is depending on its concentration of
hydrogen ion (H+) and hydroxide ion (OH-) ions that the solution contains. An acid is an ionic compound
that break apart in water to form H+’s. The higher the concentration of H+ ions in a solution, stronger
the acid is. A base is an ionic compound that breaks apart to form OH-‘s ions in water. The strength of a
base is determined by the concentration of OH- ions, the greater the concentration of OH- ions the
stronger the base. Therefore, the more H+ ions the lower the pH value is and the more OH- ions the
higher the pH values is for a certain solution. A solution is considered neutral is the concentration of H+
ions is the same as the concentration of OH- ions. The pH scale ranges from 0-14, acids are found
between 0 and 7 and bases are between the values of 7 and 14. A pH value of 7 exactly would be
considered neutral.
Vinegar has a pH value of 2.4 for it is very acidic, along with diet coke with a pH value of 3.2.
Hydrogen peroxide is slightly acidic with a pH value of 6, this is way the colour very slightly changes
away from purple. Water is neutral with a pH of 7. Baking soda and glass cleaner (Windex) are both
bases with pH values of 9 and 10.
An indicator is a special type of compound that changes color as the pH of a solution changes,
thus telling us the pH of the solution. The cabbage juice acts as the indicator in this experiment. Red
cabbage contains a water-soluble pigment in a group of compounds called anthocyanins. Anthocyanins
are found in cell vacuoles of plants, contributing to the colours of many fruits and vegetables such as
eggplant, cherries, and many berries. Anthocyanins will change colour in response to different
environment they are in. This is why red cabbage makes a good indicator.
Curriculum fit
Saskatchewan
Science 10: Chemical Reactions
Kylin Alm
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SCI10-CR2 Name and write formulas for common ionic and molecular chemical compounds,
including acids and bases
Manitoba
Senior 2, Cluster 2: Chemistry in Action
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S2-2-08 Experiment to classify acids and bases using their characteristic properties. Include: pH,
indicators, reactivity with metals. GLO: D3, E1
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S2-2-09 Discuss the occurrence of acids and bases in biological systems, industrial processes,
and domestic applications. Include: environmental, health, and safety issues. GLO: B2, B3, C1, C8
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S2-2-10 Explain how acids and bases interact to form a salt and water in the process of
neutralization. GLO: D3, E2
Grade 12 Chemistry
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C12-5-05 Describe how an acid-base indicator works in terms of colour shifts and Le Châtelier’s
principle.
Alberta Curriculum
Grade 9 Science: Unit C - Environmental Chemistry
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2. Identify processes for measuring the quantity of different substances in the environment and
for monitoring air and water quality
Science 30: Unit B – Chemistry and the Environment
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30–B1.3k Describe the relationship between pH and hydronium ion concentration