Inquiry in Action chapter 5, part 2 -- Formation of a precipitate, and neutralizing acids and bases.
We started out with a review of what we covered the week before -- that chemical changes can cause a gas to be released, a color to change, or a heating or cooling to occur. Then we worked on another possibility -- forming a precipitate.
Honestly, as I read through the lesson I thought this sounded like a massive PITA to do -- making hard water, then making soap scum. I thought it would be easier to do something like throw vinegar in milk to make curds ... but then we got into all sorts of discussions (at home) as to whether that would actually be a precipitate or a coagulate ... and then started discussing flocculation ... and never really reached a conclusion. (Side note: Rick is a chemical engineer, and tends to think discussing chemical reactions is quite scintillating.) Although it did occur to me that vinegar plus milk had a high probability of making a really stinky mess in the classroom, especially if it spilled in the carpet. So we went with the soap scum.
And, really, it turned out not to be that hard to do if you just follow the directions. I tried it all out the night before, as I usually do. The kids worked in groups, mixing epsom salts with water to make hard water, shredding up Ivory soap to m ix with the water (they sort of went wild with this part, even though I demonstrated it first, and even though we had an adult working with each group).
They were quite fascinated with the concept of pouring the soapy scummy water through a coffee filter -- most had never considered the possibility of using a coffee filter as a filter. I asked if anyone made yogurt cheese at home, since it sort of reminded me of that. I knew one family did that sort of thing (one of the adult helpers and her kids, as a matter of fact).
The kids were excellent at harvesting their soap scum off the filter, even the group that had knocked over their cup of soapy scummy water and ended up without much to use. They were really awful about putting a similar amount of soap flakes in the other cup of water -- we've gone over this concept Every. Single. Week. of controlling amounts so we only have one variable. But I guess the excitement of mucking about with the soft Ivory soap and popsicle sticks was too much for them.
They were absolute champions about making soap bubbles with a straw. Well, those with the actual soapy water -- the soap scum water was pretty disappointing.
Then we set all of THAT aside (rather regretfully, as there were those among us that thought they should play with globs of soap the rest of the day) to work some more with pH.
I had made new red cabbage indicator -- it doesn't really keep its indicator qualities if it's hanging around for a week. Plus at this point I can pretty much whip up a batch in less than 5 minutes.
The previous week one of the girls asked if we could get the color of the indicator back to the original if we mixed together the cup of indicator-plus-cream-of-tartar and the cup of indicator-plus-detergent. I had slopped them together, but it didn't work.
This week each group used a control cup, so they could see the color they were trying to get back to, and 2 other cups. They carefully added small amounts of cream of tartar and detergent powder to each. Two notes on process:
First of all, the curriculum specifies using flat toothpicks to add the powders to the indicator. I didn't have any flat toothpicks, and didn't really feel like driving around town looking for them. Instead, I cut plastic drinking straws into segments about 1.5 inches long, then split the segment lengthwise to make a little scoop. One straw gives you several scoops that are very thin (so it can get under the powder to scoop it up) and fairly spill-proof (due to the curve of the straw sides).
Secondly, when I tried this at home I originally tried using Ivory Snow laundry detergent powder. I kept dumping it in and dumping it in, trying to get the indicator to change color. It occurred to me that perhaps Ivory is mild because its pH is fairly neutral. So I switched to a small box of Sun detergent which was the cheapest thing Walmart had. Wowza, a tiny bit of that stuff made a HUGE difference in indicator color. Brand of detergent really makes a difference in how this works! And could be an interesting inquiry all on its own -- maybe a science fair project. (Of course, now I've no clue what to do with the rest of the box of this detergent, but that's another issue.)
In any case, the groups of kids successfully turned their indicators various colors, then switched around and turned them back to the original color. One group overshot it, and went to far the OTHER way. Which was actually a good experience, because then we just added a scoochy bit more of the first powder.
Afterwards everyone mixed everything together as desired, which is always one of the more popular moments of the class.