I have always remembered how cool it was the first time we made colored fire (this was WAY back when we first started homeschooling) so we decided to try it again. Sam helped since it always scares me to do things like this alone.
(People don't look extremely enthralled here, but they were!)
We haven't ton a ton of chemistry lately in school, so it was fun learning about some basics like chemical and physical changes and chemical reactions with the kids. We did a bunch of the classic demonstrations like convection currents and baking-soda-and-vinegar and hydrogen-peroxide-and-yeast reactions. (Here's a fact sheet and experiment page for that last one.)
I love their pleased smiles as the reactions work
…some of them had pleased smiles, anyway.
They liked this "fireworks in a jar" demonstration too (instructions here, but you can just google "fireworks in a jar" if that link breaks; there are tons of examples of it)
The kids really liked making these fireworks "models" out of lentils and poppyseeds and so forth. (That smiley face Ziggy made is how they really would make one of those shaped hearts or smiley faces you see sometimes at fireworks shows!) You can look at pictures of cross-sections of real fireworks to get an idea what they look like and how to make them (one is here, and we'd also watched a few documentaries about the process).
When we studied fireflies, we learned that each species uses a distinctive flash pattern to identify themselves and attract mates. So, to demonstrate this, we went over to our church gym (because it was a nice big space to run in, and because we couldn't find a place where it would be pitch dark in our house) and I brought glowsticks in three color pairs: two pinks, two blues, two greens. I gave one glowstick to each child. Then we turned out the lights, and the children that had matching colors had to find each other by flashing and waving their lights.
It was surprisingly fun. They had fun locating their "mates," but then they came up with all kinds of other firefly games too--races and relays and so forth. The glowsticks gave off just enough light that you could (mostly) avoid running into walls, but it was dark enough that everyone was laughing and squealing and having near-collisions. Good times.
Glowsticks are a pretty good representation of a firefly's bioluminescence, since they are both "cold" light created by chemical reactions. We so wish we had fireflies around here, because we love them! But at least some of us have finally have seen them in real life, when we went to Montreal last summer. And maybe someday we'll really get lucky and see some here in Utah!
Near the beginning of this unit we went through a general overview of different sources of light.
Besides the sun and fire, there's chemoluminescence, fluorescence, and bioluminescence. Glowsticks use chemoluminescence by mixing two reactive chemicals that release energy when combined. There is a good explanation here:
And a video on how you can make your own glowsticks, here. You have to have specialized chemicals and it's not super easy to make them, so it's not something we wanted to try, but the video is still interesting.
When we examined the glowsticks closely, we could see the small inner container which breaks when you bend the glowstick to activate it.
Here are some interesting videos about bioluminescence:
This one is really short and you'll want to turn down the sound to avoid the swearing. But it's cool to see algae suddenly light up blue when the water is disturbed.
This video shows someone swimming among the bioluminescent dinoflagellates—gives a better idea of how huge some of these masses are
And, since none of us have ever seen fireflies in real life, we watched this video showing what they look like (there were some in Utah a couple years ago!)
This page has a good diagram showing one way atoms make light (fluorescence), along with a good overview of what light is.
We talked about electrolysis and electroplating, and we wanted to try zinc-plating a penny as described here, here, and here. The hard part is getting the zinc for dissolving in your solution. We tried using the zinc casing from our dissected battery, but for some reason that didn't work. We could have tried to track down some hydrochloric acid and copper sulfate to use instead, but that seemed like too much trouble when we could demonstrate the concept another way.
So, first we cleaned pennies to show how copper oxide (that greenish stuff, like rust but called "verdigris," that forms on old copper) dissolves in a vinegar and salt solution. (Instructions are here.) You can clean pennies by dipping them in the solution, like this:
A half-dipped penny---you can see the difference between cleaned and uncleaned!
If you leave the pennies soaking in this solution for awhile, they get all shiny and clean. Then, if you remove them from the solution and DON'T rinse them with water, the copper quickly begins to oxidize again and the greenish-blue verdigris starts to form. You can see it clearly on these pennies above. Interestingly, this is the same chemical reaction that forms malachite and azurite!
After you've soaked the pennies, many of the copper ions have left the outside of the pennies and have become suspended in your solution. So, once the pennies are out, you can put in some small steel nails or paper clips. After awhile, the suspended copper ions (positively charged) will attach themselves to the (now negatively charged, from sitting in the solution and giving up some of their positive ions) steel nails. If you leave them in long enough, your steel nails and paper clips will end up plated with a thin layer of copper! It looks really pretty.
(unplated paper clip on the left, for comparison)
The longer you leave them in, the brighter and more noticeable the copper plating will become! These were left in for a few days.
This isn't truly electroplating, because there was no electric current in the solution to help facilitate the process. But you can see how electroplating works based on this---the current just pushes the ions off the anode and makes the object to be plated into a cathode, so it will attract those ions. Very cool!
Before we talked about rocks we talked about minerals, and when you are talking about minerals you have to talk about crystallization. I had seen a photo tutorial on how to make "hot ice" (the stuff that comes in those little hand-warmer packs; you break a disc inside and the whole packet crystallizes and turns warm) and it looked pretty simple, so I thought we'd try it. Then I read a couple other tutorials and all of a sudden it sounded like it wasn't simple at all, in fact it was fraught with uncertainty and peril, so I thought we wouldn't try it. We had several other crystallization activities, so I didn't really think it was necessary. . . and yet. I kept thinking about it, thinking how much the children would like it, IF we could get it to work.
Finally I found this video tutorial. He does a great job of making it seem possible, and giving you ideas for troubleshooting, but I also really appreciated how he said, "This is tricky to get right. You probably won't get it right the first couple times you try it."
So, trying to set expectations as low as possible, I explained the demonstration to the children, and asked if they wanted to try it, knowing it might be frustrating and it might not work. They wanted to. So we began.
First you mix baking soda and vinegar. Carefully!---since you know what happens when those two mix.
You mix till all the baking soda is dissolved and the liquid is clear. Then you heat the solution over the stove.
He explains all this in the video, but basically you are trying to concentrate your vinegar---since household vinegar is quite weak; only 5% pure vinegar or so. It makes the house smell STRONGLY like vinegar, so open a window and don't stick your nose right down by the pot, or your eyes will be watering like crazy. This concentration process can take quite awhile. I think it was about a half hour for us. You are watching for a thin, slippery film to form on top of the liquid. Again, he explains this in detail in the video, but this (above) is about what it looks like. You need to take the solution off the heat as soon as you see this skin forming. And you can see the whitish powdery stuff on the side of the pot (upper right corner of this picture, in the shadow) which is sodium acetate crystals that have already formed.
I'll leave off the explanation of the chemistry involved in this reaction---I'd probably mangle it (find it in this video or this less-detailed one)---but suffice it to say, when you drop a crystal of the sodium acetate into the supersaturated sodium acetate solution, it initiates immediate nucleation and you can actually SEE the crystals form. It is really quite amazingly cool. And, probably thanks to our meticulous adherence to the directions (and our low expectations)---we actually got it to work! And we repeated the process several different times. Once you have the sodium acetate made, you can use it again and again (you just heat it to dissolve the crystals, then start the reaction over).
It was interesting, because sometimes our crystals looked more sharp and needle-like, like this^^
And other times they spread out in fan shapes, like this ^^
So, in spite of my fears that this would be one of those really frustrating and time-consuming demonstrations that never actually works---it wasn't! And it turned out to be one of the coolest things we've ever done. Watching the crystals spread through the solution, so orderly and yet so organic-seeming, and then reaching your hand in feeling the instant heat created by the chemical reaction, is incredible. We LOVED it. We wanted to see the reaction over and over again, and to show it to everyone we knew! And we will definitely be making this again (since after many uses, we have now thrown our sodium acetate away).
Here are a couple videos---if you're only going to watch one, the first one is most impressive.
Hot ice crystallization---best version
Hot ice crystallization---slower, more needlelike crystals
My friend Andrea made ricotta and we'd been wanting for a long time to try it ourselves! And it is so easy and delicious, just like she said.
TRUE ricotta cheese, we learned, is made from whey ("ricotta" means "recooked"), but it only works with whey from a rennet-coagulated cheese. [Update: I'm wrong. It only works with whey from a CULTURE-coagulated cheese. So you can't use the whey from the mozzarella cheese to make ricotta, either, unfortunately.] In other words, you can't make this ricotta (which is vinegar-coagulated) and then make MORE ricotta from its whey. More's the pity. So this is not a true ricotta, because it begins with whole milk instead of with whey.
We tried out Andrea's recipe, and a few others, and one thing we eventually modified from hers is that the two-hour cooling time after you add the vinegar seemed unnecessary (many other recipes omitted that step). Which means you can be eating the ricotta that much sooner; yay!
Here is what we did:
In a large pot, combine a gallon of whole milk (lower-fat milks work too, from what I have read) and a pint of cream.
Heat until the milk is not-quite-boiling, between 165 and 185 degrees. Heating it all the way to 185, we read, is not really necessary if the milk has already been pasteurized, so the lower temperature works fine.
Stir in 1 1/2 teaspoons salt.
Stir in 1/4 cup plus 2 Tablespoons of white vinegar. (You can use lemon juice instead, especially if you're going to use the ricotta in a sweet recipe like cheesecake.) Watch the milk coagulate as you stir! The curds are white blobs and the whey is a clearish liquid. This is really cool to watch.
Let it sit for a few minutes and continue to coagulate.
Line a strainer with cheesecloth or a few paper towels. If you want to save your whey (it's great for adding protein to bread, smoothies, etc.), put the strainer on top of a bowl or bucket to catch the whey. If not, put the strainer in the sink. Using a slotted spoon, spoon the curds into the strainer. When you have most of the curds removed, pour the rest of the mixture into the strainer to get the last curds. The whey will slowly drain out either into the sink or into your lower bowl.
At this point you can let the curds sit for as long as you want to get the desired consistency. If you're going to eat the ricotta immediately, you only need to let them drain for 5-10 minutes! (That means you can make this ricotta, start to finish, in about a half hour!) If you want a drier ricotta for cooking lasagne, etc. with, let the curds drain for a half hour or longer.
We ate our ricotta immediately, drizzled with honey, and it was amazing!
