1.5 grapefruit (ph 4) 2 oranges (ph 4) 1 apple (ph 4) 1 small apple juice (ph 4)

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1 Purple Cabbage ph Indicator: Tasting with your eyes. Shopping list You will need enough of each food for each student to taste it as well as to make about 5 ml of juice/puree per student. Don t get caught without enough containers; I use jelly jars or 50 ml conicals for the juices and Tupperware for the samples. Budget about $50 for supplies. Lab supplies adds ~$3/student. 1/4 head purple cabbage 4 lemons (ph 2) 4 limes (ph 2) white vinegar (ph 2) 1.5 grapefruit (ph 4) 2 oranges (ph 4) 1 apple (ph 4) 1 small apple juice (ph 4) ½ watermelon (ph 6) 2 cucumbers (ph 6) 1 bag frozen corn (ph 8) baking soda (ph 8) 2 uncommon fruit (one for tasting and one to show): star fruit, lychee, cactus pear, duryian etc. Materials Overhead projector or light table (ask the classroom teacher not necessary but nice) One sheet each of pink, purple and blue paper Picture of each food item and tape Paper plates Pipette for each student as well as for each juice: (VWR# ) or a drinking straw Plate for each student (for the experiment): 6 well plate (VWR # / / etc), ice cube trays, or a white plastic plate. Prepare purple cabbage ph indicator dye. 30 minutes, can be done in advance. 1. Roughly chop a quarter of a head of purple cabbage. 2. Boil cabbage in 3 cups (~700 mls) of water for 10 minutes. The cabbage will lose its color and the water will become dark purple. 3. Remove cabbage from the infusion with a colander. 4. Store purple cabbage ph indicator in the fridge for about a week, or aliquot and store in the freezer indefinitely. Prepare samples to taste and test: ~ 1 hour, not more than a day in advance. 1. Wash and slice each food item. 2. Juice the citrus, rinsing the juicer between fruits. The ph of packaged juices may be quite different from the cut fruit for tasting. The taste-ph correspondence will be better if you make the juice yourself. 3. Purée the watermelon, cucumber and corn in a food processor, rinsing between foods. Add water to get them to a consistency that is pipette-able. The purées will be rather opaque. That s fine. Tasting with your eyes: Group experiment Arrange with the teacher to have students wash their hands prior to this activity. Give each student a small plate with a piece of each fruit/vegetable and small pile of baking soda. (Tell them to wait to taste.) Fill enough wells of your 6 well plates to test all of the samples with purple cabbage ph indicator. Place the plates on the overhead projector, light table or a white surface. Show the kids your unusual fruit and ask if anyone knows what it is. (I ve had good luck with the cactus pear as the unknown.) Explain to the students that you can tell if something is going to be

2 sour or not by the color that it turns the indicator. Have them guess what the unusual fruit may taste like. (Consider writing down these hypothesis). Tell them the indicator is purple cabbage juice. In its purified form, it is used as a ph indicator in research labs. Tape the purple cabbage picture to the purple sheet of paper. Have them taste the cabbage. Have the students taste the lemon. Add lemon juice to a well. Have a student record the data by taping the lemon picture to the (pink/red) sheet of paper. Next do the cucumber, then the apple etc. After you ve tested a few fruits, ask for predictions about the color the indicator will turn after the students taste but before you do the experiment (or the inverse). The indicator is sensitive only to ph (acidity or sourness). However, most of the foods that are not sour taste quite sweet. Use baking soda as an example to prevent students from coming away with the otherwise logical, but incorrect, conclusion that basic foods are sweet/the blue color is an indication of sweetness. After the tests are finished, discuss the results. Which fruits were the most sour? Which fruits were the least sour? Which fruits were a little sour? What color does the indicator turn when things are very sour? A little sour? Not sour at all? Now that you have many data points, have some observers look at the results and rearrange the order of the fruit pictures on the colored paper to better reflect the colors. Finally, test the unknown. Revisit the students hypothesis about what the fruit may taste like. Add the juice to a well to test the ph, record the result. With this new information, let them revise their hypothesis. Taste! Were they correct? If you are using drinking straws, rather than pipettes, demonstrate how to transfer liquid by covering the end of the straw once it is in the juice and uncovering it when it is over the plate. Hands-on activity Give the kids a chance to experiment on their own. All kids should have access to an acid (ph 2) and some baking soda dissolved in water (ph 8), along with a few of the different juices. Expect them to figure out how to titrate back and forth between acid and base and shortly thereafter, to re-discover the volcano reaction, which produces lavender colored foam and excitement. Filling 25 6-well plates with indicator takes some time, consider bringing a pitcher or large pipetman. Multiple small containers for the juices will help ensure access to reagents. Collect the materials. If there is no sink in the room, it s helpful to have a tub or a bucket for the liquid. The overall take-home message for students should be that there are a variety of ways to quantitate and experience our world. In this case, you can not only taste, but also use ph to determine food acidity. Multiple approaches help us understand and therefore manipulate aspects of our environment. Optional: Send the students home with an aliquot of the indicator (in a 15 or 50 ml conical tube), a pipette, and a note for parents. If the classroom has parent list, have the teacher send the information electronically as well.

3 Today your student did an experiment in class. We used our eyes to figure out if a food was going to be sour. We did this by mixing different fruit juices with cabbage extract, which contains a natural ph indicator. We tested to see if our predictions were correct by tasting a piece of the fruit that the juice had been made from. You can continue these experiments at home. The purple liquid in your child s backpack is cabbage extract, which is made by boiling red or purple cabbage in water for about 10 minutes. Store it at room temperature for a day or so, in the fridge for about a week or in the freezer indefinitely. Dispose of it down the drain. Mix it with juices or drop it on top of solid foods to check their ph. This works well on a white plate, or in an ice cube tray. Use a spoon or straw (with your finger on the end) to transfer small amounts of liquid. V ph scale for purple cabbage extract Today your student did an experiment in class. We used our eyes to figure out if a food was going to be sour. We did this by mixing different fruit juices with cabbage extract, which contains a natural ph indicator. We tested to see if our predictions were correct by tasting a piece of the fruit that the juice had been made from. You can continue these experiments at home. The purple liquid in your child s backpack is cabbage extract, which is made by boiling red or purple cabbage in water for about 10 minutes. Store it at room temperature for a day or so, in the fridge for about a week or in the freezer indefinitely. Dispose of it down the drain. Mix it with juices or drop it on top of solid foods to check their ph. This works well on a white plate, or in an ice cube tray. Use a spoon or straw (with your finger on the end) to transfer small amounts of liquid. V ph scale for purple cabbage extract

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