MAKING CASEIN PLASTIC
I did this in a science workshop once. The tough bit was figuring out how to make this without heat, but see, I found a way.
Take two plastic drinking cups.
Fill the first cup half full of vinegar.
Put filter paper in the second one (a coffee filter works best because it's already cup shaped).
Add two tablespoons of skim milk to the vinegar in the first cup, and stir.
Then add 1/8th Tablespoon of baking Soda to the milk/vinegar mixture. Add a little at a time.
Stir. You want just enough bubbles to fill the cup, but not have them spill over.
Scoop the bubbles you get onto the filter paper in the second cup.
Wait ten minutes.
Scoop the white gunk out of the filter paper onto a paper towel. Let it sit for a few seconds.
Then make it into a shape, and let it sit on a windowsill for a week. It will shrink and harden.
You've just made a kind of early plastic that buttons used to be made out of!
To make more plastic, use bigger cups!
RED CABBAGE JUICE CHEMISTRY
To make the red cabbage juice, just boil some red cabbage in water and bottle the liquid. It starts to stink after a few days, so you might not want to make it too far ahead of time, but if you do, that's OK kids will think it's funny!
You might want to do this experiment at the same time as the plastic one, so the ten minute wait goes faster. For this experiment, you put Red Cabbage juice in clear cups, to which kids can add different things to. Bases (such as baking powder and Baking Soda) turn it blue, and vinegar turns the juice bright red. Some things like milk don't change the colour at all, but if you also add oil, you get different coloured layers,which the kids really like. Be careful with the cabbage juice though- it is really messy and it stains.
Another experiment you can do is make slime.
You get cornstarch (made from corn, not wheat) and add a little powdered paint to it. The kids can choose which colour they want. Then add water, a little at a time, letting them mix it with their hands until it's hard when they squeeze it, but runs through their fingers when they let go. It's really messy and kids love it. Have some extra in case someone adds too much water to theirs. If you want it a little less messy, give them the cornstarch in a sandwich sized ziplock bag, and get them to add the water and paint to that. Then they can close it and squeeze it, keeping their hands clean. I'd suggest half a ziplock bag of cornstarch each, but maybe try it out yourself first to get the amount right.