Hands-On Science Activity: - by George Pastirik, President, Science Is... Copyright 2015

Cabbage Water Indicator

You will need: 1/4 of a small purple cabbage (also called "red" cabbage)

Water, cooking pot, knife, fine mesh strainer (i.e. soup/tea strainer), fork

2 litre storage bottle with cap (i.e. pop bottle), funnel

Several labeled "temporary storage containers" (squeeze bottles are ideal)

Several CLEAR jars/beakers/glasses (baby food jars are ideal), one for each material to be tested, masking tape and marker for labeling

Household materials to test: vinegar, hand soap, dishwashing liquid,

lemon juice, baking soda, washing soda, laundry soap,

pop, coffee, tea, aspirin, Windex, etc.

Adult supervision required!

WARNING: Any activity involving heat or fire can be dangerous!

Follow directions carefully and get an adult to help!

Advance Preparation:

Chop the 1/4 purple cabbage into small strips. Place in pot and add 2 to 3 cups of water. Place pot on stove and heat to simmering. Allow to simmer for 5 minutes, then shut off the heat and let everything cool to room temperature. Notice that the colouring from the purple cabbage has made the water a blue colour, and the cabbage itself is much paler than when you started. Pick out most of the large pieces of cooked cabbage with a fork and discard (or set aside for use in soup or other cooking). Pour off the remaining cold "cabbage tea" through a strainer and funnel into a storage bottle. Dilute to 2 litres, cap and label it ("Cabbage Water") until ready to use. Yes, it will smell like cooked cabbage, so you will want to keep it in the refrigerator! Also, it will not keep for too long, so only make as much as you can use within a few days.

What to do:

Keep all materials to be tested separate from each other. If they are mixed, the contamination will cause the results to be confusing. It is useful to have labeled disposable droppers, squeeze bottles and/or labeled temporary containers (like small test tubes) to hold the materials to be tested. Especially for younger children, this is more convenient than trying to pour a little bit of (i.e.) vinegar from a huge 4 litre container.

For testing powdered materials like baking soda, powdered laundry soap, etc. you should dissolve them in water first to make a solution, and have them ready in the labeled temporary containers.

Set up several clear containers (like baby food jars), and label them with the materials you will be testing. Pour an equal amount of the prepared blue cabbage water into each jar until it is about 2-3 cm (1 inch) deep. Add about 1-3 teaspoons (exact amounts are not critical) of each liquid material to be tested into the corresponding labeled container until the colour is stable. Make note of the colour changes that occur.

Cleanup: Most, if not all of the household materials used here can be washed down the drain with plenty of water. If necessary, the stronger chemicals can be flushed down the toilet one at a time, although if small amounts are used (a few millilitres / a teaspoonful), disposal should not present a problem at all.

What Happened?:

The colour of the cabbage water changed according to whether the material being tested was an acid or a base (bases are also known as an alkalis). The cabbage water also gives an indication of how strong the acid or base is. Scientists refer to how strong an acid or base is by measuring its "pH" on a scale of 1 to 14. On this scale, 7 is neutral (water), below 7 is acid and above 7 is basic. The further away from 7, the stronger the acid or base is. Here is an approximate colour list for pH testing with purple cabbage water:

pH Colour Material

1 red concentrated hydrochloric or sulfuric acid

(strongly acidic, very dangerous)

2 pink-red vinegar

3 light pink-purple lemons, orange juice

4

5

6 light purple (slightly acidic)

7 blue water (neutral)

8 light green-blue baking soda (slightly basic)

9

10 light green Sunlight laundry soap

11 green ammonia, Windex

12 green

13 yellow Drano

14 yellow lye (strongly basic, very dangerous)

Note that the colour of the original material to be tested (i.e. orange juice) can sometimes mask the actual colour change you are looking for. (Sorry, but this is not an exact science)

Additional Activities:

Refer to the labels of the materials to see if you can match similar contents with similar results of the colour indicator. For example, the vinegar container may note "contains acetic acid", and the lemon juice "contains citric acid", so they should have the same colour change. Similarly, read the labels on various cleaning products. Those that "contain sodium hydroxide" (a strong base) should all react about the same.

For younger children, try testing various fruit juices, cold coffee, cold tea and other drinks. For older children (but still under adult supervision), you may want to test other household materials like ordinary aspirin, household ammonia, Fantastik, Drano (liquid Drano is safer than mixing the granular type with water), Sani-Flush, bleach, etc. It is very important to remember: DO NOT mix the chemicals! Keep all materials to be tested separate from each other. If they are mixed, not only will the results be confusing, but there is potential danger in indiscriminate mixing of household cleaning chemicals! This includes pouring fresh material into a "dirty" container that has been used for another material. There may be enough of the first material left in the container to contaminate and possibly chemically react with the second material!

Magicians often use colour changes in their magic acts. It would be fairly simple to have several clear containers set up ahead of time with small amounts of various strong acids and bases in them, then pour cabbage water into each one successively to make it appear as if one were pouring several different coloured liquids from the same container!

Other Variables to Try: (What Will Happen If I ...)

Normally, scientists would use commercially prepared indicators like litmus paper, pH paper or pH meters to test for acids and alkalis. Many flowers and fruits contain colouring matter which you can extract with hot water and use as an indicator for acids and bases. Red, blue or purple fruits/flowers seem to work best, such as cherries, blueberries, elderberries, iris and violets. I have also heard that turmeric (an ingredient in curry powder and sauce) can also be used as an acid-base indicator.

This is a good start for a science fair project. Many advertisers (especially soap and shampoo manufacturers) say their products are "pH neutral". With what you have learned here, you can test the products to see if this is true.

 

 

Cabbage Water Indicator (Parent/Teacher Page):

A note about nomenclature: We refer to "purple" cabbage, although if you go to the produce department of your grocery store, it is often called "red" cabbage. Oddly enough, the "cabbage water" that is made from it is a pretty blue colour, not purple or red.

 

Applications to the Alberta Science Curriculum:

Grade 1: This activity can conceivably be tied in with the "Creating Colour" Topic, BUT take note that the Learner Expectations for that topic are more directed at the properties of (for example) artist’s materials. The "mixing" component of this activity is potentially very confusing to the student, especially if you consider mixing blue paint and yellow paint to get green paint. The confusing part to the student in this activity would be seeing a clear liquid added to a blue liquid to get a green liquid! Given the student’s experience with mixing paints, this is not the expected result! So use caution in how the mixing aspect of this activity is perceived by the students. Regardless of the potentially confusing aspect of mixing colours, this activity (perhaps done as a demonstration rather than a hands-on activity) would give students the opportunity experience many Specific Learner Expectations of the "Creating Colour" Topic. Students will learn to identify colours, compare and contrast colours, group similar colours, observe transparent vs. opaque colours, and see that colour can be extracted from some materials.

Grade 2: This experiment is applicable to the science inquiry Units on "Exploring Liquids", although explaining the detailed science behind the different pH levels may not be necessary.

Grade 3: If you stretch it, this activity could be used with the "Testing Materials and Designs" Unit. Although that Unit concerns testing building materials, this cabbage water activity could give the students some practice in "testing materials" and recording their results.

Grade 5: This activity is VERY applicable to the "Classroom Chemistry" Unit, as we determine the properties of several household substances. As well as determining if a substance is acidic or basic using an indicator (a Specific Learner Expectation), we see how some substances dissolve to form solutions, see some examples of simple chemical reactions and we also see how chemical reactions resulting in colour changes are very useful to the scientist.


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