Please note that in a class of students it is likely that one of them is at least partially colourblind (1 in 12 males are colourblind). As this is an activity distinguishing colours, these students will not be able to tell some colours apart and perceive some colours differently, although the activity will be no less interesting for them. The common red/green colour blindness means reds and greens (or colours containing reds and greens such as browns) look similar. More information at colourblindawareness.org and colorblindguide.com/post/the-advantage-of-being-colorblind.
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. The dye can be stored in the fridge for a week) There is a physical change as the dye moves out of the red cabbage pieces into the hot water.
Pour off the red cabbage dye into bottles.
Making an infusion in hot water, like this, is one way that plants are prepared for use. Here are other indigenous ways of reparing medicinal plants:
| Preparation method |
Description |
| Paste |
Fresh plant parts are crushed to obtain a paste used externally or internally. |
| Poltice |
Plant parts are crushed to obtain a soft moist mass generally used externally to treat swellings, pain, inflamed or infected body parts. |
| Juice |
Obtained by squeezing or crushing plant parts and filtering through cloth. Sometimes requires addition of freshwater or other liquid for dilution. |
| Powder |
Obtained by crushing dried plant parts. |
| Chewing |
Fresh plant parts are chewed without prior transformation. |
| Infusion |
Plant parts are plunged in either hot or cold water for several minutes. If hot water is used infusion is taken as a tea. More than one plant species can be used in conjunction. |
| Decoction |
Plant parts are boiled in water for several minutes and the extract is used. More than one plant species can be used in conjunction. |
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 (attached below). The lower the pH, the more acidic. The higher the pH, the more basic (or alkaline). Students can try individual liquids to test, and then go on to test mixtures of liquids. Their results can be graphed for discussion of which materials are more acidic and which more basic, as well as other patterns that come out of the graph (e.g. outliers). Students can also mix household liquids to find the emergent pH. Chemists, biologists use the pH scale to test chemicals and water for how acidic or basic they are.
Optional for older students - explain the pink/purple/blue colour changes in terms of molecule structure:
Show students the structure of two dye colour molecules (see red blue pigment molecules file attached) - one is the colour molecule when cabbage dye is pink or red, the other the colour molecule when the dye is blue . Challenge them to spot the difference between them (one of them has an extra white hydrogen atom top left). This tiny difference between the two molecules makes them different colours - in acids there are extra hydrogen atoms around and the dye molecules gain a hydrogen, so turning the dye pink or red. In a base, the dye molecules lose a hydrogen atom and so turn blue. When the dye is purple (at neutral pH, between an acid and a base) there is a mixture of red and blue dye molecules, so making the dye purple.
The reason the red cabbage dye turns green at high pH values is due to another pigment molecule, an anthoxanthin, which is colourless until high pHs, when it turns yellow. The mixture of the yellow anthoxanthin and the blue anthocyanin at high pH values gives the green colour.
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 added to them which combines with the sourness to make a pleasant mixture.) 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 yarn - use acid/base to vary the colour of larger batches of red cabbage dye and add yarn to it (mordanting the wool before hand should make the dye stick better - see notes). Very gently squeeze extra dye out, then put individual yarn pieces in little pots to dry. Do not rinse with water as the dye will come out. Make contrasting colours then twine to make a bracelet.
Dyeing cloth - add to batches of dye or drip acid/base onto a piece of fabric dyed with the purple red cabbage dye. Lay cloth out on cardboard to dry.
Plant dye extraction activity: extraction of dyes from plants including native plants.
