Red cabbage indicator colours

I bought a red cabbage a couple of weeks ago as my son had been asking to do some colour changing with it again.  He used the cooled cooking liquid as a pH indicator, watching it change colour to pink in vinegar (an acid) and blue when he added bicarbonate of soda solution (alkaline), as he's done many times before.  I froze some of the cooking liquid in ice cubes for another day and had a think about how we could do something a little bit different - he wasn't bored of the same experiment, but it was getting a bit repetitive for my liking!

I decided he might like to try an experiment where he wouldn't know exactly what was going to happen, testing the pH of a range of household substances with the indicator.  To get a good range of colours, I chose things which ranged from very alkaline to fairly acidic, so this comes with a safety warning - he wore childrens' goggles and I made sure he didn't spill the most alkaline one on himself.  You could do a version with a smaller range of pH if you don't have the goggles or are less able to supervise very closely.  The substances I put out for him to test were:
- tap water
- fruit-flavoured water (contains citric acid)
- hand soap diluted with water
- vinegar
- kitchen cleaner diluted with water (this one is very alkaline)
- bicarbonate of soda dissolved in water
- lemon juice
- milk

Liquids ready for pH testing

I put each of these in an empty small yoghurt pot and made them roughly the same volume.  I also gave him 8 frozen blocks of red cabbage indicator (I froze it in a fish-shaped ice cube tray, hence they are all fish shaped) as this gave him the same volume of the indicator to add to each substance.  He's pretty good at squirting with a pipette, but measuring out the same volume for each would have been tricky and he'd have likely got frustrated with it.

Adding the frozen red cabbage liquid

The activity for him therefore comprised talking about what was in each pot - I labelled them, so he tried to read the label, and getting him to guess what colour the indicator would go when he added it.  He knew what would happen with lemon juice, vinegar and bicarbonate of soda, but he was less sure with the others.  After we'd talked about what he thought might happen, he carefully added an ice cube of indicator to each, with me helping him to ensure he didn't splash the kitchen cleaner.  I'd optimistically put out some kids' tweezers for the purpose but he discovered that curved ice cubes which are slightly melting are very slippery and he used his fingers instead after deciding that the tweezers were unnecessarily frustrating!

Indicator changing colour

The ice cubes started to melt quite quickly, and we were able to see a hint of the colour change pretty much straight away.  We watched together as the colour spread out from the ice cube (which itself remained purple until it melted).  He was quite fascinated by this, and he watched for a few minutes before deciding that they weren't melting quickly enough and we should leave them for a while.  

He returned to see the colours, with the kitchen cleaner turning green, bicarb solution blue, various shades of purple from hand soap, milk and water, and then various shades of pink with lemon juice, vinegar and fruity water.  We talked about which were acids, which were neutral and which were alkaline.  He was most surprised that the kitchen cleaner went green as he hadn't seen the indicator go green before, and we discussed how this can hurt his skin and eyes, whereas the soap for cleaning his hands is fairly neutral.  He wanted to arrange them in colour order, so we did that carefully, and then left them on the table as he wanted to show his Dad later in the day.

Colour change from pinks through to green

It was a nice variation on exploring pH with red cabbage, with a bit more of an element of discovery for a change and a nice colourful outcome.  It seemed to go down well with my son, but I'll have to wait and see whether he asks to do it again!