Red Cabbage Indicator investigation

Red cabbage indicator - full of awesomely awesome awesomeness.

Crash course in acid base chemistry for those of you playing along at home:

Acids are substances that release hydrogen ions. Bases are substances that accept hydrogen ions. No need to worry about those last two sentences if they scare you, the investigation will still be awesome. Water soluble bases are called alkalis, so in everyday situations the terms 'base' and 'alkali' are pretty much interchangeable.

pH is a measure of how acidic or basic a substance is. pH 7 is neutral - by definition pure water is exactly pH 7. The more acidic it is, the lower the pH, so anything below 7 is acidic. The more basic it is, the higher the pH , so that's the above 7s.

pH scale facts:
- The numbers in pH actually mean something - there is a reason that 7 is neutral. It's not an arbitarily assigned number like many of the other scales that we use.
- Each increment means ten times - so something at pH 5 is ten times more acidic than a substance at pH 6. A substance with a pH of 4 is one hundered times more acidic than the pH 6 substance.
- The 'p' on pH means something, and so does the 'H'. The p is always lower case even when at the beginning of a sentence, and the H is always a capital.

An indicator is a special substance which changes colour depending on what pH it is. One of the coolest and easiest indicators - especially for little investigators - is red cabbage indicator. it has very distinct colour changes, is dead easy and you don't need to worry about harmful chemicals poisoning the kidlets.

Preparing the indicator

Get a red cabbage. You don't need much - I was making heaps for a large group, but if you're just doing it for a few people then a quarter of a cabbage will be heaps. Chubs wasn't keen on the whole cabbage, she wanted the pretty half one, so I had to swap the half for the whole when she wasn't looking. Hopefully you can avoid this - for less that one hundered kids then a quarter cabbage will be heaps.

Roughly chop

Chop the cabbage roughly. You want to allow the red colour to 'leech' out.

Cover with hot water

Bung it all in a saucepan and cover. Heat it for a while (about 10 - 15 minutes, no need to be fussy)

Soggy cabbage

When the colour has started to run out of the cabbage and it looks 'washed out' then it's done.  

Strain

Strain it out into a tub or bowl.

Yuck cabbage

It feels very wasteful, but I ditch the cabbage each time. I'm sure there's a recipe to do something with it, but it does seem a bit like a Dickensian work house. If anyone has any ideas, let me know!

The investigation

One of the great things about this investigation is that you can just get in and play. It can be as structured or as relaxed as you want it to be. As long as you are using foodstuffs then it's totally fine for little hands and mouths. If using anything that might sting eyes then be careful, but I'm guessing if you're reading this then you had a toddler and you don't need me to tell you that.

The RCI will stain hands like beetroot, but it's not harmful and will wear off quickly. Clothes staining can be a bit hit and miss - but this is a good one for play clothes anyway. It can fizz a lot - so it's messy and wet, but easily cleaned. Best bet is to do it outside where you can just hose everything off.

Here's a few samples that I tried this afternoon. A bicarb solution (sodium carbonate or sodium hydrogen carbonate, depending on which brand), vinegar and water is more than enough to get you going for a while. Milk, lemon juice, toothpaste suspension, apple juice, sugar solution, dishwashing liquid and anything else you can find make great things to test.

RCI will turn to different colours depending on the pH. Most items which are toddler safe are in pH 5 - 9, so green, blue/purple and pink will be the colours you see the most.


For this investigation, I added the RCI first - these are just RCI.

RCI only

I added some bicarb solution to one, more water to the next one, and then vinegar to the last one.

 

With samples

One of the great things about indicators, is that they aren't like dyes or paints. If you mix them together, then they will keep changing to the colour that the solution is. You can keep mixing the substances together and it won't end up as one brown murky mess like paint or food colouring would. For the pHs of normal foodstuffs, RCI will change between pink, blue, purple and green.

Vinegar and RCI in the bottom glass (pink)
About to add bicarb and RCI solution from the top

In the photo above, I added the bicarb solution from the previous photo to the vinegar one.

It fizzes like bicarb and vinegar normally do
Be careful if you're not expecting this.

 

Once the carbon dioxide has fizzed off, you can see that the RCI is still pink (so acidic) but it's less pink that it was. This is to be expected; the bicarb just neutralised it a bit. If you keep adding more bicarb (either solution or powder) then it will become more purple/blue and then green.

Keep mixing, keep trying things out, keep investigating. It's lots of fun! Make a table with your results and be very organised, or just keep playing. Lots of fun - enjoy.