You step into the kitchen and come upon a curious sight. Three jars sit on the counter. Blue, purple, pink. The fluids flow between each jar over a color-stained paper towel bridge.
The air smells a bit foul and pungent like stinky cabbage.
Call Sherlock Homes, we have a mystery!
How did this happen?
Is it possible that the purple can be separated into blue and pink?
Or did blue and pink combine to make purple?
There is only one way to know for sure! SCIENCE!
Do your kids love mysteries like mine? Do the love or loathe science? No matter how you answered those questions, I think they will love this at-home experiment that I created to teach kids about pH and the properties of water. (Inspired Paper Towel Rainbow and red cabbage pH indicator).
Supplies:
- red cabbage
- Ziploc sandwich bag
- 1 quart mason jar with lid
- 3 small glasses or jars (I used Bonne Maman jam jars)
- lemon or lime juice
- baking soda
- 2 paper towels
- water
- patience
Instructions
- At least a few days before your experiment: chop up or slice a Ziploc sandwich bag’s worth of red cabbage, put in the Ziploc sandwich bag, and place in freezer.
- This will last in your freezer for several months, so if you’re a mom like me who puts stuff in the freezer with good intentions but then get caught up and forget about it, you’ll be fine.
- Freezer burn should not cause problems.
- The minimum amount of time needed is uncertain, but you do want it to freeze through. The freezing action breaks the cells to release the color-changing magic (saving you from stewing your cabbage on the stove! You’re welcome.
- If you forget to put it in the freezer, cook it on the stove and strain the water into a jar. Just let it cool before using with your kids.
- The day before your experiment: put the frozen cabbage in a 1 quart mason jar and cover with water. Place in the refrigerator overnight.
- The day of your experiment:
- Place 3 jars on the table.
- Fold each paper towels into thirds lengthwise and place one end of the towel in the middle jar and fold over the lip of the jar into the adjacent jar on the end. Repeat for the other side.
- Pour about a tablespoon or two of lemon or lime juice into one end jar (I put the acid on the right jar)
- Put about 2 teaspoons of baking soda in the other jar, add 2 tablespoons of water, and stir to dissolve
- Fill the middle jar with red cabbage juice — it’s wise to decant the liquid and hold the cabbage back using the lid or another method.
- Now wait to see what happens.
Quickly, you’ll see the purple (pH neutral) juice turn into bright pink in the acid jar and bright blue in the base jar!
The longer you let your jars sit, the closer the liquid levels will get to each other, reaching equilibrium if left long enough.
Why does cabbage juice turn different colors?
The chemical responsible for the color change, anthocyanin, takes on different configurations based on the pH of the water, which our eyes read as different colors!
Compound Chem has a great visual for you to learn more.
What is pH?
Liquids can be basic, neutral, or acidic. Each term refers a an amount of hydrogen ions (H+) or hydroxyl ions (OH-) present in a water-based solution.
Acids are chemicals contain hydrogen that is release as hydrogen ions (H+) when dissolved in water. With higher amounts of hydrogen ions in water, red cabbage juice will turn pink.
Basic or alkaline chemicals have hydroxyl ions (OH-) in their structure, so when added to water, the OH- is released, reducing the relative concentration of H+ and turning cabbage juice blue, green or yellow.
Neutral liquids have an equal number of hydrogen ions and hydroxyl ions.
Acids and bases can be weak or strong. Strong bases will turn cabbage juice yellow, weak bases will turn it blue. Strong acids turn cabbage juice red. Weak acids are more magenta or pink.
How is the water moving between the jars?
Water is an amazing molecule! It is known as a polar molecule because the hydrogens and oxygens do not sit in a straight line, they are bent like a boomerang with the oxygen on one side and both hydrogens on the other side.
This polarity makes the water molecule act like a weak magnet to other objects and to other water molecules, attracting them to each other. This attraction means that when a water molecule is adsorbed into the paper towel, more water molecules follow through cohesion as they get pulled up the paper towel against gravity – like a water train!
The water moves up out of one jar and then down into the next jar where the acid or base is waiting on the other side!
Extensions
What happens if you put a paper towel between the pink and blue jars or pour the liquids together?
If you used baking soda – you can pour some acid into the base and watch it bubble like a hot tub, or erupt like a volcano if you pour a lot of it in (maybe do this in your sink!).
If the amount of acid and base is properly portioned, this process should neutralize them and turn the color back to dark purple.
What can you use to make more colors in the rainbow?
First off, do not mix anything other than baking soda and vinegar or juice.
It is hard to come by strong acids and bases in the home, for good reason, but here are some items that you can find around the house to add to cabbage juice to determine the approximate pH:
- ammonia from cleaners (DO NOT MIX WITH ACID)
- soda/pop or any other carbonated drink
- vinegar
- lye
- masonry cleaner
- washing soda
- antacids
- cream of tartar
Can you change the color using just your breath?
Use a straw to blow air through the neutral purple juice. What color will it turn? Why?
(hint: we exhale carbon dioxide)
What makes sour candies taste sour?
Dissolve a sour candy and a not sour candy in cabbage juice and find out!
What other questions can you think of?
zuzuberry 