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Matter Lab 6-8 / Week 09 / Wednesday
3/6
Week 09 · Acids, Bases and a Cabbage Indicator

Wednesday

Bench Lab: The cabbage indicator
// Sorting kitchen liquids with a purple juice and the pH scale
⏱ about 20 min

Wednesday: Bench Lab: The Cabbage Indicator

A bowl of purple liquid sits in the middle of the Bench. Rocket peers into it from a safe distance. "Cabbage juice," he says. "This is the secret?"

"Anthocyanin," Raven says. "The dye in red cabbage. It changes color with pH."

She pours a little into five cups. Rocket adds vinegar to the first. The purple blooms red.

"What do you notice?" Raven asks. "Red for acid. Now the soap water."

The next cup turns green. Rocket whoops. Nova hovers over the row, her light cycling through the colors.

"Red, purple, green," she says. "You have a scale. What would you need to turn colors into numbers?"

"Something with known pH to compare," Rocket says, already reaching for the plain water.

Your mission

You will make a red cabbage indicator and use it to sort five kitchen liquids into acids, bases and neutral.

Hot water pulls the dye out of cabbage leaves. The solution is red when acidic, purple when neutral, and green to yellow when basic.

That description comes from a university chemistry text. Your job is to see it for yourself and record the colors.

  • A quarter of a red cabbage, chopped by a grown-up
  • Hot water, poured by a grown-up
  • A heat-safe bowl, a strainer and a clear jar
  • Five clear cups, labeled: vinegar, lemon juice, baking soda water, soap water, plain water
  • Vinegar, lemon juice, baking soda, dish soap and water
  • A teaspoon, a tray, a towel, safety glasses or sunglasses, and your Bench Log
Safety first
Wear safety glasses or sunglasses. Liquids can splash.
A grown-up handles the cutting and the hot water. The hot bowl stays where the grown-up puts it until it cools.
Nothing is tasted or smelled up close. Nothing goes in the mouth.
Only kitchen liquids are tested. Never a cleaner, bleach, ammonia or drain cleaner.
Cabbage juice stains. Cover the Bench and wear old clothes.
Pour leftovers down the sink with running water and wash your hands.
  1. Grown-up: chop a quarter of a red cabbage and put the pieces in a heat-safe bowl.
  2. Grown-up: pour hot water over the cabbage until it is covered. Let it sit for fifteen minutes to cool.
  3. Strain the purple liquid into a clear jar. This is your indicator.
  4. Make baking soda water: stir half a teaspoon of baking soda into half a cup of water.
  5. Make soap water: stir two drops of dish soap into half a cup of water.
  6. Pour two tablespoons of indicator into each of the five labeled cups.
  7. Add one tablespoon of the matching liquid to each cup. Stir once with a clean spoon each time.
  8. Record the color of every cup in the data table, then sort them into acid, neutral and base.
CupColor after mixingAcid, neutral or base?My pH guess (low, 7, high)
Vinegar
Lemon juice
Baking soda water
Soap water
Plain water
PREDICT BEFORE YOU POUR
  • Read the question.
  • Tap your answer.
Vinegar contains acetic acid. What color do you predict the cabbage juice turns?
Baking soda is a base. What color do you predict?
Plain water is neutral. What do you predict?

Reading your results

Vinegar and lemon juice should turn the indicator red. They contain acids: acetic acid and citric acid.

Baking soda water and soap water should turn it green. Both are bases.

Plain water should stay purple, the neutral color. If your tap water shifts a little, it may not be exactly neutral.

You have just used a property, indicator color, to identify a class of substance. That is the evidence this week's standard asks for.

What the lab showsTrue or false?
Red cabbage dye is red in an acid.?
Soap water turned the indicator red.?
Purple means neutral.?
The indicator is a tool for sorting liquids by pH.?
WHY THIS EXERCISEThe three colors are the whole code: red, purple, green.
The dye in red cabbage is called ____.
In a base, the cabbage indicator turns ____ to yellow.
How many of your five cups should turn red? Type a number.
Draw your five cups in a row and color each one the way it looked. Write acid, neutral or base under each.

Beautiful colors. Tomorrow you will read a table of real pH values and place your cups on it.

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