We've used baking soda and colored vinegar on the light box using squeeze bottles.
We've used baking soda and vinegar to make our volcano model erupt with a bit of unintended realism.
This time I wanted them to work their fine motor skills more. Using eyedroppers is excellent for strengthening and reinforcing the pincer grasp which is a great pre-writing activity. It also has the side benefit of making your vinegar and baking soda last a lot longer. I sprinkled a shallow layer of baking soda in a glass baking pan and gave them small containers of vinegar and and eyedropper each.
I gave Ava red and blue vinegar so she could mix purple. I gave Michael red and yellow vinegar so he could mix orange. They had a lot of fun with the activity and I had to rinse and refill the baking pans three times for each child.
Then I ran out of baking soda. They poured the rest of the vinegar into the pans to watch the colors blend and use up the last of the fizz. Then we noticed some undissolved baking soda at the bottom of the pans. The children began to draw in the baking soda. Michael wrote his name. Ava helped me spell hers. Then after some shaking and swirling, the names disappeared and they could begin again. Michael drew a snowflake and Ava drew a smiley face with arms and legs. They continued to draw and shake away several more creations before wandering away.
It was an accidental discovery, but drawing in the leftover baking soda and vinegar was a perfect additional activity for my pre-writing goals. Next time, I would have them use the eyedroppers rather than spoons for the drawing activity. This would also be a great way to have children practice writing their letters.
You should also be able to recreate this activity by stirring salt or sugar into colored water. You just have to be sure to keep adding sugar or salt until no more can dissolve and you have a layer left in the bottom of the pan. It would be even more fun on a light box (see how we made some out of storage bins).