I got this fun idea from About.com
They always seem to have the neatest ideas on all kinds of subjects.
5 clear glasses (those wine glasses you don’t use anymore now that you have kids are a great idea)
food coloring – blue, green, red, yellow
tablespoon – measuring spoon and flatware
patience – lots
Place 1 tbsp of sugar in the first glass, 2 in the second glass, 3 in the third glass, 4 in the fourth glass. Leave the fifth glass empty.
Add 3 tbsp of water in each glass and stir the sugar water mix. If all the sugar doesn’t dissolve, add one (or two) more tbsp of water to all four glasses.
Add 2-3 drops red food coloring to the first glass, 2-3 drops yellow to the second glass, 1-2 drops green to the third glass, and 1-2 drops blue to the fourth glass.
Pour some blue sugar water (the most dense) into the fifth glass (about 1/4 full). Now, very carefully, add the green sugar water to the fifth glass. Next using a straw, you can place a straw in the green sugar water, almost to the bottom. Then, by holding your finger tightly over the top of the straw, you can capture the sugar water into the straw. Then you can transfer the solution to the fifth glass, Then you can take the straw and very slowly let up on the pressure of your finger as you drizzle it over a spoon perched ever so carefully on the blue layer.
Now, as the volume decreases in your green sugar water glass, you may need to employ a slightly different technique (particularly if you end up doing this experiment twice). I recommend the following procedure (and this is the only time I would ever recommend this type of technique because we are working with sugar water; Parents, if you aren’t comfortable teaching your child this bad lab technique, do this part for them – or if you happen to have a bulb pipette laying around, you can use that :)). [Caution - never pipette by mouth any sort of chemicals. It's an incredibly dangerous practice. You could cause a chemical burn on/in your mouth or ingest something toxic or deadly]. But since we are dealing with foodstuffs, and we drink many sugary drinks by straw (cola, lemonade, etc), this not totally inappropriate in this situation.
Take your straw and suck up some of the green sugar water, but be careful not to ingest it (it’s sugar water, so it’s not the worst thing you could ingest, unless of course you’re diabetic or you are allergic to food dyes). Quickly (and I mean very quickly), remove your mouth and place your finger tightly over the end of the straw. This will hold the liquid in the straw. You may want to practice a bit before you transfer the solution to the final glass. Then you can take the straw and very slowly let up on the pressure of your finger as you drizzle it over the spoon perched ever so carefully on the blue layer.
Repeat this for the yellow layer and then the red layer. This is where patience comes in handy.
I recommend this method because my first attempt at doing this ended up very, very badly as you can see from the picture below (I did something very, very wrong):
Instead, if you follow my inappropriate for the lab but okay for potable drinking solutions, you may end up with something resembling a rainbow like we did. Or, if you don’t have a straw handy, see the * below
I’m pretty sure that if we lightened up the amount of blue and green dyes we added to the sugar water, we would have seen a better result. I always add too much blue and green dye. Separately, I really like the intense colors, but for purposes like this, lighter is almost always better.
I really have fun with our at home science experiments. My oldest 6 seems to really enjoy it a lot as well. She did all of it up to the final step of making the layers. She even told me, “mom, look, the sugar is dissolving!” Hmm…I guess some of the things I teach her have sunk in.
Science terms to talk about with this experiment (depending on the age of the child): density, density column, concentration gradients, color mixing (especially since we had no green food coloring, so I asked my daughter, “how can we make green color from the colors we do have”?), dissolving, solutions, dilute, concentrated. I’m sure there’s more, but that’s a few of them.
* I have just heard from the author of the rainbow in a glass project at About.com and she told me that if you don’t have a straw handy, you can follow this procedure:
You can pour the solution over the back of a tablespoon or even a wooden spoon without disturbing the layers. The large surface area helps to prevent messes. Just be sure the spoon is touching the side of the glass and that it is just above the surface of the liquid.