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.
Pick a petal from the flower (use one rose petal, one camellia petal, 4 bluebell flowers, or equivalent).
Tear the petal into small pieces and put them in the mortar.
Add one teaspoon of water. Grind the petal and water together with the pestle: push down while grinding in a circle. Keep grinding until the water is as dark as the petal. It’s important that you get the water really dark.
(If you are not using a mortar and petal, tear the petal into pieces and put in a baggie with the water, then smash and squish the petal in the baggie until the petal juice is dark.)
Suck up the petal juice with a dropper. Put a few drops of petal juice in each well of the tray. Add a drop or two of acid (vinegar) to one well of the petal juice in the tray. Add a drop or two of base (baking soda solution) to another well. What new colours do you see? Are any of them familiar flower colours?
(acid makes the petal juice pink/orange; base makes it purple/blue (and green with some flowers).
Experiment with adding various amounts of acid and base to the petal juice.
Can you reverse the colour changes?
Ask students to record the changes they find, or visit the groups and record their results on one board (organizing the colours as they are reported). In class discussion, distill out the most frequent colour results in acid (oranges and pinks) and in base (blues and purples). White flowers can stay white in acid and turn yellow in base. Some colours will not change (generally yellows, oranges).
Just like you can make different colours by adding acid or base, some flowers are red, purple or blue depending on the levels of acid or base in their petals. They contain colour molecules (pigments) called anthocyanins that change structure slightly depending on the amount of acid or base they are in - one structure is red and the other is blue. Depending on the mix of red and blue anthocyanin molecules, the colour can vary between pink/red/purple/blue (all the colours you saw in the activity), giving rise to a great variety of flower colours from one kind of pigment molecule.
Optional - show students molecule models of red and blue anthocyanin molecules (I used the cyanidin molecule, which is the red pigment in dark red roses), and challenge them to find the difference between them. (Clue: look at the white hydrogen atoms.) One particular hydrogen atom on the cyanidin molecule is present in the acidic version of the molecule (which is red) and missing in the basic version (which is blue). Depending on the amount of acid or base, there is a different ratio of red and blue cyanidin molecules, which gives rise to the range of red-purple-blue colours.
You may have also made green petal juice. This is when, in base, one kind of pigment molecule (anthocyanin) turns blue and another pigment molecule (anthoxanthin) changes from white to yellow. When the yellow mixes with the blue anthocyanin, green results. If the colour changes are grouped on the board as they are gathered, the two kinds of pigment molecules can be seen separately and as mixtures in some kinds of flower petals.
Not all flower pigment molecules change with the amount of acid or base e.g. the yellow of tulips and other flowers. Pigment molecules mix and match together to make all the different flower colors that we see.
Students freely experimenting may also notice that bubbles sometimes form. If acid and base are added to the same well of the tray they chemically react to make CO2 gas (see baking soda chemistry).
Flowers attracting pollinators focus
By varying acidic/basic conditions in their petals, the anthocyanin molecules in flower petals make red, blue or purple colours. By mixing the anthocyanins with other colour molecules (e.g. yellow, orange), flowers can display a wide variety of colours. Different pollinators are attracted to different coloured flowers. See the flowers and insect pollinators activity. Some flowers even change colour as they age (e.g. the forget-me-not), indicating to pollinators they are past pollination time, so the pollinator will move to another flower which is ready for pollination.
Physical and Chemical Changes focus
Discuss how tearing and crushing the petal is a physical change. The shape has changed but it is still the same molecules of dye and water. Crushing the petal in water is a physical change: the dye moves from the petal into the water, but the molecules stay the same.
Ask students to look for chemical changes when they add acid and base to petals. Chemical changes are shown by: a change in colour, a change in smell or appearance of a gas. Students should find colour changes as the petals change colour in acid and base, as well as the appearance of a gas (when baking soda solution and vinegar are mixed together).