We have two big cherry trees in our yard and most years, the Spring Snow Gods willing, we can look forward to eating cherries right off the tree around the Fourth of July. This year was particularly abundant and we have been feasting on cherries for a couple of week now. So have the birds. Aubrey was commenting on the partially pecked leftovers and I told her to hang on to those and I would show her a cool science experiement which is now a also blog entry.
As it turns out there is a group of chemicals called xanthocyanins (zan • tho • sigh • ann • ins) that are contained in a large group of fruits and vegetables. They are also naturally occuring acid/base indicators so what we have are pH indicators that can be made from ordinary, right-out-of-the-grocery-bin, blueberries/strawberries/rhubarb/plum/etc.. When the juice from said fruit or vegetable comes in contact with a strong base, like ammonia or bleach, it changes color dramatically. Hit the solution with a bit of acid, like vinegar, and the color changes back. Who needs a ticket to Disneyland?
1 Your Choice of Xanthocyanin Source
Blueberries Strawberries Rhubarb
Cherries Raspberries Blackberries
Radish Swiss Chard Red Grapes
Eggplant Turnips Beets
Pomegranate Plums Boysenberries
1 Kitchen knife 4 Baby food jars
1 Votive candle 1 Oven Mitt
1 Book of matches Water
1 Vinegar 1 Ammonia or Bleach
1. Using the kitchen knife, chop the fruit or vegetable into very small pieces. The smaller that you make the pieces, the more surface area there will be to react with the water and extract the indicator.
2. Cover the bottom of one of the baby food jars with chopped organic matter and add water until you just cover the top of the pile.
3. Light the candle and heat the bottom of the baby food jar in the candle flame.
4. Heat the jar until the water turns a color, usally purple to dark blue. Remove the jar from the flame, which you can now extinguish, and pour equal amounts of the liquid into each of the three empty baby food jars.
5. Using a straw, add a couple of drop of ammonia to one jar, clean the straw in water, add a a couple of drop of ammonia to the second jar, clean the pipette, and add vinegar to the third jar. Observe what happens to the color of the liquid.
How Come, Huh?
Indicators are extremely large, complex molecules. By adding a base to the red cabbage juice indicator, you were raising the pH of the solution, and a ton of hydrogen ions were lost. When this happened, it changed the shape of the molecules.
When you change the shape of a molecule, you can also change the way that molecule captures and reflects light. In this case, the original shape of the red cabbage molecules reflected reds and purples with a couple of blues thrown in. When the hydrogen ions went on vacation, the molecules contorted into different shapes and instead of reflecting reds and purples, they reflected yellows, greens, and a blue or two.