Mystery message and blueberry juice

A few weeks ago, we found that blueberries made a nice pH indicator. The liquid from cooked, blended and strained blueberries turned blue in an alkaline solution and bright pink in acid. This is, in itself, a nice experiment activity for children but I've been wanting to try a bit if something that you could consider forensic science for a while. My son is not yet 4 so I wanted a simple mystery/puzzle for him to solve.

Hidden message

It's not exactly a gripping plot, but the mystery was that someone had written a letter to a Duplo lady in invisible ink. My son had to use the blueberry liquid to reveal the message, and then to test a sample of invisible ink from lots of Duplo people to work out who sent the letter.

Revealed message - photo taken a few minutes later after it had gone blurry

The invisible ink was bicarbonate of soda dissolved in a little water, and the message said 'thank you'. He was pretty heavy-handed with the blueberry and the message was a bit blurry after a few seconds as the bicarbonate of soda on the page dissolved into the coloured solution. Still, with a bit of help pointing out the letters, he sounded out the message. It's worth saying that you need to find paper this works on - the IKEA drawing paper we have keeps a pretty similar colour change compared to in solution but some paper appears to be quite acidic and the colour change is less noticeable. An alternative 'mystery' scenario would be that some kind of liquid (a drink or some bodily fluid!) was found at a crime scene and you have obtained samples from people to test.

Who wrote the message?

Anyway, once he'd read the message I explained that the lady wanted to know who had sent it (for older kids it could be a more sinister message!). I produced a paint palette with samples from lots of Duplo people sat around it - you could just as easily omit the people and number the samples but given his age I wanted to make it engaging. The samples included one of bicarbonate of soda solution as was used to write the message and others which were either just water or varying concentrations of vinegar.

Adding blueberry pH indicator

I gave him a pipette and more of the blueberry liquid and he decided he was going to squirt a little of the blueberry pH indicator into each well of the palette. It went different shades of pink until he got to the bicarbonate of soda when it turned deep blue! He checked all the other samples (which I wasn't expecting, I thought he'd decide he'd found the person and stop!) and then declared that he'd found the person. He picked the Duplo man up and took him to the lady by the message!

It was the man in the yellow hat!

He wanted to do it again immediately, which was a good sign that he enjoyed it! I didn't have another message (poor advance planning on my part), but I agreed I would set up another palette. I found a substitute Duplo person and filled one well with bicarbonate of soda solution again. I was lazier this time and just put water in the other wells so they all turned the same shade of pink, but it's worth saying that you do only need about 6 blueberries and half a teaspoon of bicarbonate of soda to set this up. He enjoyed it a second time around, and wanted to find his Dad (who is working from home due to Covid-19) to tell him about it. I think I will have to get creative again soon with pH indicator, possibly with red cabbage which we've used before as you just use the cooking liquid and eat the rest.