Cabbage juice chemistry

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Cabbage juice chemistry
Age Range 5-8 (grades 1-2), 8-11 (grades 3-5), 11-14 (grades 6-8), 14-18 (grades 9-12)
Group Size medium (6-20)
Setting indoors
Setup Time medium (6-30 min), long (31 min - 24 hrs)
Duration medium (11-30 min)
Subjects chemistry
Key terms acid, base, neutral, alkaline, solution, diluted
Materials a copy of the periodic table of the elements, 10 or more small clear glass vials with lids (one for each chemical plus one for the control), 9-11 clear glass jars with lids and labels (one for each chemical), measuring cup, measuring spoon, two buckets (one for dumping and one half full with water for rinsing), distilled water for mixing with chemicals, baking soda, vinegar, salt, alcohol, ammonia, aspirin, vitamin c, lemon juice, antacid tablets, small bottle of sprite or other clear liquid, tap water, white plastic laminated strip or a white paper towel to put vials on for accurate color comparisons, jug of water from boiling purple cabbage
Presentation Style demonstration, interactive, lecture, model, student led

Brief Description

Students will compare the whether common household products are acids, bases, or neutral. The experiment uses purple cabbage juice as an indicator. The cabbage juice changes color depending on what type of household product is added to it.

Scientific Background

  • Everything in the universe is made of chemistry, so a chemist studies everything. Some things are made of just one chemical element, some have a few combined together, and some things are very complicated combinations of chemical elements.
  • The Periodic Table of the Elements is a list of the most basic chemical elements that we know about. It is like the alphabet in that all things are made by combining these chemicals together. Water is made of just two chemical elements: hydrogen and water, so we say it is H2O or two atoms of hydrogen and one of oxygen to make one molecule of water. Other things, like a person, are very complex combinations of chemicals.
  • A chemist studies what everything is made from by learning about these elements and finding out how they combine into molecules. Chemists make guesses about things and then find ways to test their guesses to see if it was right or wrong. Wrong guesses can be as good as right guesses if you learn something from them. A chemist also has to be very careful how they combine things together. Some chemicals are very safe to study and some are very dangerous! We will study our chemicals in a safe way today.
  • One of the important things chemists learn when they study the world around them is that many things are either acid, alkaline (base) or neutral. An acid and an alkaline are opposites, like black is the opposite of white. Neutral means exactly half way in between the two. It is important to know this when you are studying chemistry, but sometimes it is hard to guess. Some foods, medicines and cleaning products are acid, base and neutral. If a food is acid, it will taste sour, like pickles. Never try to taste medicines and cleaning products! Cabbage juice helps you test things because it turns a different color when it is mixed with an acid, a base or a neutral, and it is a safe way to find out what something is.
  • Knowing how this works helps people everyday. We use baking soda and something acidic like buttermilk or lemon juice to bake cakes or cookies. We could also use baking powder for the same thing. It is a mixture of baking soda and the dry powder called cream of tartar, that becomes tartaric acid when mixed with water. Geologists use this knowledge to figure out which rocks are which. Limestone and calcite will bubble up when acid drips on them and quartz or chert will not.

This Is How You Play

How to make the cabbage juice: Prepare this ahead: ½ a head of purple cabbage, cut up into large chunks, covered with water. Bring to a boil and simmer gently for 10 minutes. Strain out cabbage, cool and store liquid in a glass or plastic bottle in the refrigerator or the freezer until the time comes. Save the cabbage to show the kids. Some like to eat it too. HINT: Cook the other half cabbage and freeze the juice until you need to do this program again. Give it time to thaw over night because it doesn’t do well in the microwave. HINT: test a tablespoon sample of your cabbage juice before doing it with the kids. If it is too concentrated the purple color will take a lot of the chemical sample to change color and it will be harder to see the color. If it is too diluted the purple color will look almost pink from the beginning and the difference will be less dramatic.

Set up: One vial has plain cabbage that never changes, so we can compare things with it. This is called a control. It is an important thing for scientists to always have a control. Label your control vial to remember what it is. Each of the other vials has an equal amount of cabbage juice. Set them in a row on a white background with the store container of a variety of common household chemicals set behind them. (This list is fairly harmless. The only one that the kids have to really watch out for is the ammonia. Discuss the importance of safety when using any chemicals.) Also set out the labeled jars next to each chemical. Pour a small sample of each liquid chemical into its labeled jar. For the dry chemicals, mix it with distilled water. It will take some time for the aspirin and antacid tablets to break apart and dissolve so set this up at least ½ hour ahead of time. Set a straw next to each chemical jar and set out the color cards for comparison.

Procedure: Talk about acid, alkaline and neutral chemicals, how the whole world is made of chemicals, and how scientists have to find ways to test things safely and carefully to learn about what everything is made of in the world. One way to test is with purple cabbage juice because it will change color. Alkalines turn it green or blue, acids turn it pink and neutrals don’t change it. Ask the kids what color they think each chemical will turn the cabbage juice. If you have enough vials, each kid can have two to do a test. Go one at a time, letting them choose a chemical to test. Have the whole group observe each test and guess what they think will happen. Help them to think it through as much as possible before the test, getting a vote and some discussion. Make sure they understand that the way a scientist thinks is not about being right or wrong, but making guesses and then testing out their guesses. It is OK to make a wrong guess because that is how you find things out. Then, the kid who is doing that test takes the straw, dips it into the jar of chemical solution, puts their thumb over the end and lifts out a sample to drop it into their vial. (Some kids may need some help learning how to do this.) See what color change happens, then decide the degree of acid, alkaline or neutral it is by comparison with the control and with other vials.

Often I will put out a dish of cooked purple cabbage. I usually make it with a little vinegar, salt and maybe some butter and caraway seeds. The curious kids will want to taste it. Have a trash bag handy if they want to spit it out. Some will really like it, some will think its ok, some will taste it and spit it out and some will not taste it at all.

Materials: a copy of the periodic table of the elements, 10 or more small clear glass vials with lids (one for each chemical plus one for the control), 9-11 clear glass jars with lids and labels (one for each chemical), measuring cup, measuring spoon, two buckets (one for dumping and one half full with water for rinsing), distilled water for mixing with chemicals, baking soda, vinegar, salt, alcohol, ammonia, aspirin, vitamin C, lemon juice, antacid tablets, small bottle of sprite or other clear liquid, tap water, white plastic laminated strip or a white paper towel to put vials on for accurate color comparisons, jug of water from boiling purple cabbage



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