Fizzing and colour changing cabbage

My mum used to make really nice pickled red cabbage when I was a child, and I decided to buy some a few months ago, but it was oddly sweet. It's sat in the fridge ever since as nobody else in the house likes pickled anything. I've been trying to think of new uses for things around the house to amuse my sons during lockdown, and I realised a fun science experiment was sitting unloved in the fridge!

Pipetting vinegar from a jar of pickled red cabbage

We've played with red cabbage before as a pH indicator - the anthocyanins which give it colour change from pink in acid to blue in alkali with various shades of purple in between. I've boiled red cabbage and used the leftover liquid for our experiments before, but the pickled red cabbage has the anthocyanins in the vinegar used to pickle it. This has the potential for a fun colour changing twist on the fizzing (because it produces carbon dioxide) reaction between vinegar (acetic acid) and bicarbonate of soda which we've previously done in film canisters to make them fly like rockets

We tried it a couple of different ways. My son likes transferring liquids with his pipette and using the funnel on his conical flask from our science set, so I first gave him two solutions.  One was some of the red cabbage coloured vinegar from the pickle with about half the volume of extra white vinegar added to increase the volume (the colour is very vivid so this doesn't make it too pale to be fun and I wanted to make sure we had enough to repeat the experiment if he decided it was fun).  The other was a teaspoonful of bicarbonate of soda dissolved in about 80ml water.  I told him he should choose one of the liquids and transfer it to his flask, so he picked the red cabbage coloured one and we talked about how it was pink.  Next, he poured the bicarbonate of soda solution in through the funnel, which has the effect of slowing the rate at which it pours in.  There was a nice fizz, and the solution changed colour to a bright purple.

Colour changing and fizzing

I thought we could make it more fun by adding the red cabbage in vinegar to some bicarbonate of soda powder.  This is more like the reaction we did in our film canisters, and produces a very fast reaction as the solution is more concentrated.  My son was keen, and this time I got him to pour the coloured vinegar into the flask onto the bicarbonate of soda, with his brother watching from his high chair.

Pouring the coloured vinegar onto bicarbonate of soda

It worked really well and there were so many bubbles of carbon dioxide produced quickly that it bubbled out of the flask.  The littlest one and I thought this was really cool, and my eldest son got applause from his brother, but he was somewhat perturbed that it had overflowed and insisted that I clean it up!  It was on a plastic tray as I'd anticipated the mess, and once I'd wiped up the overflow he marvelled at the deeper purple (almost blue) colour of the contents of the flask and we compared it to the original pink.

An impressive colour changing fizz

He got over his dislike of the mess, and asked to do it again with his Dad watching, but he didn't want it to spill over!  I put a bit less of both bicarbonate of soda in the flask and gave him a little less vinegar and explained that I'd done this - he seemed to accept that the amount of bubbling was related to the amount of the acid and base that he was going to mix.  He duly performed the experiment for his new audience, and happily I'd reduced the amounts enough that it was impressive but not messy! He also discovered that if he swirled the flask he could make it fizz some more as there was still some bicarbonate of soda stuck to the bottom, and the colour changed to something closer to dark blue.

Colour change - with bicarbonate of soda added on the left

This wasn't a great time-filler, but it was a fun experiment to do with both fizzing and colour changing.  The jar of red cabbage certainly had greater potential than I'd thought when I bought it to eat as you'd normally throw the vinegar away once you've eaten the cabbage!