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Matter Lab 6-8 / Week 09 / Bench Day
6/6
Week 09 · Acids, Bases and a Cabbage Indicator

Bench Day

Bench Day: Cabbage rainbow
// Sorting kitchen liquids with a purple juice and the pH scale
⏱ about 20 min

Bench Day: Cabbage Rainbow

Saturday, a jar of leftover cabbage indicator sits on the Bench beside a row of eight clear cups.

"Rainbow challenge," Rocket says. "Each person picks a kitchen liquid. We line the cups up from most acid to most base."

Raven pours indicator into every cup. "What do you notice before we add anything? All the same purple. That is our starting point."

A grown-up adds a splash of lemon juice. Bright red. Rocket adds baking soda water. Green.

Nova hovers over the row. "Two cups look almost the same red," she says. "How could you tell which is more acidic?"

"Compare them to something with a known pH," Rocket says. "Or get pH paper."

Raven lines the cups up, red to purple to green. "A rainbow," she says, "made of evidence."

  1. Acids such as acetic acid in vinegar and citric acid in lemons turn blue litmus red and react with bases.
  2. Bases such as soap and baking soda (sodium hydrogen carbonate) feel slippery, turn litmus blue and react with acids.
  3. The pH scale runs from 0 to 14. Seven is neutral, below 7 is acidic, above 7 is basic, and each step is ten times.
  4. Red cabbage dye is an indicator: red in acid, purple when neutral, green to yellow in base.
  5. Only kitchen liquids are tested. Cleaners, ammonia and drain cleaner never come to the Bench.
◇ FAMILY CABBAGE RAINBOW
Pour leftover cabbage indicator into a row of clear cups, two tablespoons each.
Each person chooses one kitchen liquid from the allowed list. Vinegar, lemon juice, baking soda water, soap water, sugar water, salt water or plain water.
Add one tablespoon of each liquid to its own cup. Line the cups up from reddest to greenest.
Talk it over: which liquids are acids, which are bases, and which left the purple alone?
For a grown-up
Only the listed kitchen liquids go in the cups. No cleaners of any kind, and never two products mixed.
If more indicator is needed, a grown-up chops and pours the hot water, as on Wednesday.
Safety glasses on, cups on a tray, and everything is rinsed down the sink afterward. Cabbage juice stains.
Acid, base and pH facts this week come from the CK-12 chemistry text at chem.libretexts.org and the USGS Water Science School (usgs.gov).
Safety first
Wear safety glasses or sunglasses.
A grown-up handles any cutting and any hot water.
Nothing is tasted or smelled up close. Wash hands when you finish.
READ THE RAINBOW
  • Read the question.
  • Tap your answer.
Sugar water left the indicator purple. What does that tell you?
Two cups are both red. What would tell you which is more acidic?
Where does the greenest cup belong on the pH line?
The dye in red cabbage turns which color in an acid? Type one word.
WHY THIS EXERCISEOne color, one family. That is the simplest indicator rule there is.
Draw your family's row of cups in order, colored as they looked. Write the pH line underneath with 7 in the middle.
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