Two-coloured invisible ink

I bought another red cabbage last week, so have been thinking of some different ways to use the purple cooking liquid.  In our usual indicator experiment, we use red cabbage to show us whether a liquid is acidic or alkaline.  This works because red cabbage contains anthocyanins, and the proportion of different forms of the anthocyanin molecules (which appear different colours) depends on the pH. In an acidic environment the red cabbage liquid turns bright pink and in the presence of an alkali it is blue/green.

Red cabbage is so pretty, even before we use the leftover anthocyanins as indicator.

We've recently experimented with different ways to use turmeric for revealing invisible ink.  The curcumin in turmeric turns purple when it's added to an alkali, but remains yellow in the presence of an acid.  This means that you can draw a hidden picture with alkali (bicarbonate of soda solution) and your child can paint with curcumin solution to discover the hidden picture. We had lots of fun with this, and a less messy alternative of dyeing cotton with turmeric and painting pictures with bicarbonate of soda solution and making them disappear again with acid (vinegar).

I thought it might be possible to harness the greater colour changing ability of red cabbage liquid to reveal a two-coloured invisible ink picture with a purple background.  This would work if it was drawn with both as acidic and alkaline invisible inks.

I had a go at drawing a picture with both acid (spirit vinegar) and bicarbonate of soda solution on paper, letting it dry, and then getting the small boy to brush red cabbage cooking liquid left over from dinner over it.  The bicarbonate of soda turned it blue where it was painted.  However, the vinegar gave a very feeble colour change and the pink didn't stay localised to where the vinegar was drawn on the paper. I tried with kitchen roll in place of paper and the pink was brighter (I suspect because the pH of the paper was a little lower), and it was nice to see both colours and for him to tell me where he'd found acid and alkali, but because the paper is so absorbent, all the liquids spread a long way and it didn't really work for visualising a picture or hidden message because it was all so fuzzy.  I thought that there must be a way to make it work, and eventually - after testing various types of paper and cotton wool pads, as well as experimenting with different acids - I hit upon a combination that worked nicely...

The best combination I found was to use concentrated citric acid solution (I'd bought some citric acid to make the bath fizzy, and used a very small amount of this) and concentrated bicarbonate of soda solution for the two types of invisible ink. I checked with some pH paper, and the citric acid solution was about pH 2 whereas the vinegar I'd tried was only pH 4, so there is a good reason why the citric acid worked more effectively. The other key to success was to use fairly absorbent paper; IKEA's children's drawing paper (sold as MÅLA in the UK) was the best we had, but shiny white printer paper didn't work well. I drew the picture with a cotton bud dipped in the 'ink', but a fine paintbrush would work just as well.

I've joined the #mathmondayfun team on Instagram and this week's theme for play is 'can you find a number'. I thought we could use the two colour invisible ink for revealing hidden numbers (as well as some other pictures I'd drawn of things he likes).  For our numbers, I decided to make use of the two colours to distinguish between odd and even numbers.  Odds and evens are something we've talked about quite a bit recently as the small boy enjoys spotting house numbers when we're out and about, and our local streets are all numbered with odd houses on one side and evens on the other. I also painted some dice spots, again with the acidic ink for odds and the alkaline for evens. The small boy enjoyed discovering the numbers and telling me if they are odd or even, and counting the spots then matching them to their corresponding numbers. He needed a little encouragement to paint all 12 pieces of paper (in hindsight maybe too many!) but he got there after about 20 minutes! By the time he'd finished, some of the early ones were going a little blurry, but overall it worked pretty much as I intended and led to some nice counting, conversations about numbers and about acids and alkalis.

Revealing the hidden numbers

Odds are pink, evens are blue/green

It felt like this was a nice extension to our previous invisible ink play, with a bit more potential for creativity as well as science. I suspect we will come back to this again pretty soon, although if only the small boy would actually eat the red cabbage, I'd be tempted to buy it more often.