OK, so really it's chemistry, but the littlies prefer to think of it as magic.
At this time of year, most weekends we have a big roast and frequently red cabbage is on the menu; usually I use any leftover cooking water to add into stock, but last time we decided to save it up and freeze it into icecubes for a little experiment - red cabbage indicator solution!
So, we set up 3 glasses with warm water in them; one we left as is, one had a teaspoonful of bicarb (baking soda?) dissolved in it, and one had a teaspoonful of citric acid - though lemon juice would work just as well.
We added a cabbage juice cube to each glass and watched them thaw ...
We determined that acidic solutions turned red, alkaline ones turned blue, and neutral ones were somewhere in between, and they were all very pretty.
If you wanted to do this properly you ought to cook the cabbage without vinegar (which is acidic) as that does alter the pH of the indicator solution a bit - so the neutral water probably wasn't entirely neutral - but you can get bogged down in detail with tiny people I think, and all they really needed to see was 3 different colours.
We did some neutralisation experiments - what happens if we mix the acidic and alkaline solutions together? What if we add some baking soda to each glass? Or some more citric acid? I will confess, the washing up bowl was employed for some large scale chemistry.
At this time of year, most weekends we have a big roast and frequently red cabbage is on the menu; usually I use any leftover cooking water to add into stock, but last time we decided to save it up and freeze it into icecubes for a little experiment - red cabbage indicator solution!
So, we set up 3 glasses with warm water in them; one we left as is, one had a teaspoonful of bicarb (baking soda?) dissolved in it, and one had a teaspoonful of citric acid - though lemon juice would work just as well.
We added a cabbage juice cube to each glass and watched them thaw ...
We determined that acidic solutions turned red, alkaline ones turned blue, and neutral ones were somewhere in between, and they were all very pretty.
If you wanted to do this properly you ought to cook the cabbage without vinegar (which is acidic) as that does alter the pH of the indicator solution a bit - so the neutral water probably wasn't entirely neutral - but you can get bogged down in detail with tiny people I think, and all they really needed to see was 3 different colours.
We did some neutralisation experiments - what happens if we mix the acidic and alkaline solutions together? What if we add some baking soda to each glass? Or some more citric acid? I will confess, the washing up bowl was employed for some large scale chemistry.
For those wanting to know the science, red cabbage (and the juice!) contain an anthocyanin whose structure changes depending on the pH, causing a colour change. It's the same kind of molecule that makes hydrangeas pink or blue depending on the pH of the soil. You can get more details here.
Or you can just enjoy making pretty colours!
Or you can just enjoy making pretty colours!

