You are here

Activity

Red cabbage dye (and pH indicator)

Summary: 
Prepare red cabbage juice and make it turn colours with acids (e.g. vinegar) and bases (e.g. baking soda). Use it as a pH indicator, to measure how acidic or basic something is.
Science content (2016 curriculum): 
Biology: First People’s sustainable use of Living Things (K, 2, 3)
Chemistry: States of Matter, Properties of Materials (K-7)
Chemistry: First People’s Materials/Separation methods (1, 6)
Chemistry: Physical Changes, Solutions, Mixtures and Separating (2, 4, 5, 6)
Chemistry: Chemical Changes (2, 7)
Materials: 
  • red cabbage leaf
  • scissors, or sharp knife and chopping board
  • heat proof container, ideally transparent
  • boiling water from kettle
  • optional: sieve or strainer
  • spoon
  • squeeze bottles
  • white ice cube tray or paint tray
  • vinegar
  • baking soda in water (1 tspn in 1 cup)
  • other household liquids and powders to test, light coloured or white e.g. lemon juice, apple juice, shampoo (dilute 1:5 in water), milk, water, dish soap (dilute 1:3? in water), ammonia (careful - demonstration only) and straw pieces to use as pipettes
  • optional: different kinds of water to test e.g. tap water, pond water, ocean water
  • optional: cotton or other natural fibre to dye
Procedure: 

Chop/cut the red cabbage leaf into small pieces, and put in the heatproof container.
Pour boiling water over the red cabbage pieces.
Let the cabbage pieces soak in the hot water for 5 mins or more, while stirring with the spoon. (The longer it soaks, the darker the dye will become. Overnight will give the darkest dye.)
Pour off the juice (or strain). This red cabbage juice dye can be stored in the fridge for at least a week. (Or leave it in the fridge with the cabbage pieces still in it, and the dye will get darker.)

Red cabbage dye as an acid-base or pH indicator
Aliquot the dye into squeeze bottles for adding to separate wells of a white tray (or use the spoon).
The dye turns pink when an acid is added to it, and blue if a base is added to it. Something that is neither acidic or basic (e.g. water) will leave the dye purple.
Try adding household materials to the dye, to find out if they are an acid or a base. Best are those that are colourless or a light colour, so they don't mask any colour change in the dye.
To more accurately determine the pH of household materials, use the colour spectrum on the pH testing worksheet. The lower the pH, the more acidic. The higher the pH, the more basic (or alkaline). Chemists, biologists use the pH scale to test chemicals and water for how acidic or basic they are.
Extension: Relating to sour foods
Foods that are sour are acidic, so you can find out if something is sour without even tasting it! If it turns the dye pink it will be sour. (Some sour foods e.g. lemonade have a lot of sugar to them which masks the sourness a little.) See sour foods activity for more detail on testing foods with red cabbage dye.

Red cabbage as a paint or fabric dye
Painting - paint the juice onto paper and let dry, then paint on it with acid/base with Q-tips.
Dyeing cloth - crush cotton cloth or yarn in red cabbage dye. Use acid/base to vary the colour the red cabbage dye first, or drip acid/base onto a piece of fabric dyed with the purple red cabbage dye. Lay out on cardboard to dry. Do not rinse with water as the dye will come out, as this method did not use a mordant.
First Nations extract dyes from many native plants e.g. many kinds of berries (purple, red, blue colours), Oregon grape stems (yellow) and red alder bark (red) and lichens (green/yellow/brown).

Notes: 

Try breathing through the cabbage juice - CO2 in your breath makes it slightly more acid. http://www.amybrownscience.com/2012/03/red-cabbage-ph-indicator-in-respi...
Add windex (base), nail polish remover (acid), mouthwash (base)

This activity was a take-home for Tyee Elementary School (see attachment).

To dye cloth more effectively, use a mordant e.g. alum.
Other First Nations dyes at http://www.scientistinresidence.ca/pdf/life-science/Soils,%20Plants,%20a...

Paper label to go under ice cube tray in Maple Grove folder.

Grades taught: 
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 6
Teacher: 
Brenda Koch
Fiona Laporte
Gretchen Bartlett
Ingrid
Lisa Shideler
Sonia Ko
Sonja Watson
Teaching site: 
Aboriginal Focus Elementary
ingridscience afterschool
Maple Grove Elementary
Laurier Elementary
After School Program at Elementary schools in New York City
Activity originally developed and delivered: 

The School at Columbia