Cabbage Chemistry
Acid-base indicators provide a great platform for a variety of at home chemistry experiments that anyone can do. One of the simplest indicators that is readily available is red cabbage. It turns out that the colored pigment that gives the cabbage it color is a natural acid-base indicator. The red color of cabbage comes from a molecule called anthocyanin. This naturally occurring dye changes it color depending on the the presence of an acidic or alkaline (basic) substance.
Many other foods contain anthocyanins including cranberry juice, black currants, and strawberries. Some flowers such as hydrangea also contain anthocyanins, and this makes their color sensitive to the acidity of the soil in which they grow.
To create an indicator solution you need a few things:
- one head of red cabbage
- a blender (the easiest method)
- a knife and a 3 quart pot
If you have a blender, grab three to four leaves of cabbage, break them up into reasonable size chunks and put in a blender. Add a few cups of water and blend until the chunks of cabbage turn to small bits. About thirty seconds should be sufficient. Strain the blended liquid through a screen and you have your acid-base indicator solution.
If you are using the pot method, chop up a few leaves and toss into a pot of water. Bring to a boil for about 20 minutes (enjoy the aroma) then allow to cool. Strain the solution through a screen and you have your indicator.
Now the fun part is to find some items from your kitchen to test with your indicator. To test some liquids or powders, first add a bit of your indicator solution to a glass of water. Then you can add a bit of say, lemon juice, orange juice, baking soda, egg whites, etc. Watch carefully to see what color the liquid changes. You may want to prepare several containers of indicator solution so you can test and compare multiple items.
What to do with the left over cabbage?
You can mix up a huge batch of indicator solution for your friends or classmates, or you could cook up some sauteed cabbage or maybe even make a sweet and sour German dish. In anycase, don’t let the cabbage head go to waste! Here’s a quick listing of many dishes you can cook up with your leftovers.
pH values for common household items.
Normally an acid (lemon juice) will change the cabbage indicator to a bright red color and bases (like washing soda) will change the color to a light green or yellowish color. The colors correspond to what chemists refer to as the pH of the solution. Things with a low pH value are acidic and those with a high pH value are basic. Below is a sample of some typical values of pH for some household items.
- 1.0 battery acid (sulfuric acid)
- 1.8-2.0 limes
- 2.2-2.4 lemon juice
- 2.2 vinegar (acetic acid)
- 2.8-3.4 fruit jellies
- 2.9-3.3 apple juice, cola
- 3.0-3.5 strawberries
- 3.7 orange juice
- 4.0-4.5 tomatoes
- 5.6 unpolluted rain
- 5.8-6.4 peas
- 6.0-6.5 corn
- 6.1-6.4 butter
- 6.4 cow’s milk
- 6.5-7.5 human saliva
- 6.5-7.0 maple syrup
- 7.0 distilled water
- 7.3-7.5 human blood
- 7.6-8.0 egg whites
- 8.3 baking soda
- 9.2 borax
- 10.5 milk of magnesia
- 11.0 laundry ammonia
- 12.0 lime water
- 13.0 lye
- 14.0
Values taken from the CRC Handbook of Chemistry and Physics
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