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Purple Cabbage Juice as an Acid/Base Indicator

Acids and Bases rates right up there with the word Chemicals to make people feel intimidated. Well, all substances are chemicals. If you read the chemical rundown of dihydrogen oxide, better known as water, you can feel a little apprehension. Essentially, chemicals, acids and bases are substances. Some of these substances are dangerous, some of them we eat, some make up our body, some have no effect (inert).

You can buy a powder that can be reconstituted as purple cabbage juice. However, it is fun to boil a few purple cabbage leaves in the classroom for students to see or boil the leaves at home for quick purple cabbage juice. If you don't think you have enough, simply dilute the juice. As long as the purple color is present, you can get a color change. I've boiled cabbage juice and placed it in used plastic soda bottles and sealed with the soda bottle lid. It held up fairly well. My intent was to freeze it for later use. Cabbage does stink when it cooks. When the juice sours, it is particularly offensive. I do recommend refrigerating if you are going to use within 2 weeks or freezing for much later use. Hey if someone drinks it, it is certainly not poisonous. They will just be shocked that something so joyous and purple tastes like, well you know, cabbage.

This is how I set up the lab.

Students did their own experimentation. However they use materials as a group. I have them record their results on a table.

Purple cabbage juice was in a container made for pouring. Substances to be tested were in labeled cups with eyedroppers. Students were given clear cups or test tubes to do tests. I had students measure 5 mL to pour in the test containers. Then they; were to add one of the substances to observe what happens. A neutral substance remains purple, a base turns blue or green and an acid turns pink.

Safety key: Always dilute any substance that is harsh as much as possible and still be effective.

Substances I used,
lemon juice, milk, water, orange juice, clear soda, diluted dishwashing liquid, diluted laundry detergent, baking soda, salt, rubbing alcohol, tomatoe juice, diluted ketchup, diluted syrup. pepper, rosemary

Conclusions and Questions:
1. What is the advantage of putting data on a table?
It takes up less room and results are easier to see.
2. Which substances were acids? List substances which turned pink.
3. Bases are usually bitter. Why do you think that is true?
Bases are usually poisonous and this is nature's or evolution's way of preserving a species. You don't eat what is bad for you. You can discuss with students that artificial chemicals produced in a lab may be very dangerous and taste good. An example is ethylene glycol better known as antifreeze. Small amounts, less than a teaspoon, kill many dogs and cats every year.
4. What color did bases turn? blue or green
5. What is a pH scale? A scale from one to 14 that rates acids and bases.
6. . What is the pH of lemon juice? 2 Why is lemon juice not dangerous like hydrochloric acid even though it has a low pH? Lemon juice is categorized as a weak acid and hydrochloric acid is a strong acid. A strong acid dissociates into ions more completely than a weak acid does.
7. Why do you think hardware stores sell hydrochloric acid under the tradename of muriatic acid?
People would not buy something they knew was extremely dangerous. Muriatic acid is dangerous but people do not have the association of danger like they have to hydrochloric acid.

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