Here are a couple of really fun art projects to do that involve a little "chemistry".
Concentration: Water color with salt: Have your child cover a page in watercolor paint. He or she can make a specific picture, or be more abstract about his or her work - it really doesn't matter much. Then, sprinkle kosher or sea salt over the painting. This will draw the excess water toward the salt clumps, creating a cool effect. If your child is old enough to try to explain why this happens, have him or her look up solutions chemistry to answer the question, "why did it do that?"
Solids and liquids: Mix Sea Salt, glue and sand together into three cups. Drop a few drops of paint into each cup (one red, one yellow, one blue) and stir it all adding ingredients as needed until it is a thick paste like consistency. Give your child a piece of tag board and a fat craft stick and allow him or her to "scrape" the pasty stuff onto the tag board. Encourage your child to make globs of color and to mix additional colors right on the page. Let him or her have fun with if for awhile. Then let the artwork dry. It should have a beautiful texture.
Oil and water don't mix: Use an oil pastel to help your child draw an outline drawing. Something like the outline of a fish. Alice used a fish here, but there are many other pictures that would work too (if you don't have pastels, crayons work too. Just explain that crayons are made of wax which acts the same way as oil. After the drawing is done. Have your child watercolor over the drawing and point out how the watercolor moves away from the "oil" on the page.
Acids and Bases: Put a piece of thick, non-glossy paper in the bottom of an art tray, or cookie sheet that was raised edges. Watercolor paper works best. Sprinkle a light layer of baking soda over the top of the sheet of paper. Allow your child to drop color onto the baking soda. You can use a paintbrush and water color, or you can use food coloring droppers. The color drops should be somewhat spread out with white space between them. Then give your child a small bowl of vinegar and a pipette, thick paintbrush stick or spoon from which to drip the vinegar onto their color drops. then let them go at it while they watch this chemical reaction take place. Your child is likely to want to do this more than once but if you lift the paper before it dries the color will run. Have another sheet and tray ready nearby if you want to allow for "seconds". Let the paper dry and then spray it with a sealant to keep.