Ingredients and equipment
citric acid powder
white sugar crystals
finely chopped purple cabbage
hot water (made on request by an adult)
lots of containers
paper with a list of all the white powders
This could be done on another day, if the kids haven't already done it. Add the cabbage to some water. Stir it a bunch. Strain it with the sieve. Try out what happens when you mix it with practically everything reasonably safe that you think of in the kitchen. Orange juice, lemon juice, apple juice, soap, shampoo, citric acid, baking powder, baking soda, vinegar, milk...you name it!
I gave them an unsorted list showing what powders they had, a box with nine compartments of powder, a pile of chopped cabbage, and access to me for questions about material properties and another adult who wasn't doing computer work for hot water on request.
(I'm so glad we happened to have purple cabbage in the house! I think making the indicator ate up half an hour for them. It hadn't actually been part of my plan at all.)
They've both played with cabbage-water indicator before, so were able to spot the baking soda and citric acid almost right off, from how strongly they affected the indicator. Neither, it turned out, knew how jelly was made, although both knew it was ground-up horns, skins, and hooves!
Both were very keen to make their own jelly, as well, and improvised it using powders C and H (sugar and gelatine) off their own bats.
Material properties used in identification
Rice flour: Not terribly basic or acidic. Doesn't dissolve. Doesn't form a non-newtonian fluid with water. (I was rather hoping they'd try cooking some of the harder ones, since rice flour gives a very similar result to cornflour then, but they didn't.)
Baking powder is the only one that fizzes a lot with water.
Baking soda fizzes a lot with vinegar and turns purple cabbage water turquoise.
Citric acid fizzes a lot with baking soda and turns purple cabbage water very pink.
Salt and sugar both dissolve well and don't affect the indicator. Taste tells the difference. To distinguish these without tasting, either have an adult heat them both in a pan on the stove until the sugar browns but the salt doesn't, or use a microscope to tell cubic salt crystals from columnar sugar crystals. (We just tasted.)
Icing sugar and cornflour form non-newtonian fluids, but only one dries into icing, or taste will tell these apart again.
Gelatin dissolves in hot water but not cold. It's kind of brownish, too. Maybe use brown sugar crystals instead of ground rice, or add brown sugar as a tenth powder? Or get hold of white jelly crystals or agar powder. Agar gels on contact with cold water, too, which might make things easier.
Warnings for supervisors
It's important to prep the kids on washing up properly between tests or using separate containers and spoons for each test.
I think it's good to explain that the reason there is no safety equipment to speak of is that all the compounds are known to be safe without glasses or gloves. Also prep on how tasting kind of ruins the point until you've got down to only two options, and on writing down results of tests so you remember what you were doing.
Had to have a wee talk with one of them about how labelling one's trials is good, but labelling other people's crockery with vivid without asking is not so good.
I really loved the little delegations that came to me to inform me of my errors.
- They thought I'd forgotten to prepare the indicator for them because I just handed them a pile of chopped cabbage.
- They thought I'd put in nothing but citric acid for all the powders, because they were adding vinegar to the indicator each time instead of water! A question about why they were doing it showed them what was wrong with this approach.