Are you looking for a good, easy, affordable science fair project that will engage your child in learning and engage the audience at the science fair? Here are some ideas:
You can make a Chladni plate with a metal can or plastic cup, a balloon, and tape. Put it on top of a speaker and sprinkle some salt on the balloon. When you play music, the sound will vibrate the salt. (You could also use a guitar or harp to show how the string vibrates when played, or use a tuning fork to demonstrate sound vibration, or have other kids make craft stick harmonicas.) The tutorial includes historical notes and information about the science involved.
First, I implore you not to use vinegar and baking soda and call it a volcano. This chemical reaction has nothing to do with the geological processes involved in a real volcano, and the more we do the “volcano experiment” the more we perpetuate confusion for our kids. Try one of these activities instead:
The Jar of Bubbling Goo
Make ice cubes from a paste of baking soda and water. Then fill a jar with oil and vinegar. Shake the jar to show how they are immiscible liquids. Add a few drops of food coloring. Shake it. It will bond to the water, but not to the oil. Add the baking soda ice cube. It will react to the vinegar, creating gas bubbles that rise to the surface, bubbling through the oil.
Alka-seltzer tabs contain citric acid and baking soda / sodium bicarbonate (a base). They’re inert till mixed with water, then combine to make a carbon dioxide gas. You can use this reaction to inflate a balloon, or make a zip-lock bag explode, or launch a film canister rocket (outside!) or make exploding paint bombs as shown in the video. Learn how all these alka-seltzer reactions work.
Make your own weather station tools and use them to measure the weather. You’ll find directions for a rain gauge, thermometer, weather vane and anemometer (shown in the video) at this DIY Weather Station post.
Color Mixing Lightbulbs
Most of the other projects are super cheap. This one isn’t… it’s about $80. But pretty cool. Use a red, green, and blue LED to show how colors of light mix. (Learn how.)
The Nervous System
I think you could do something with a model of the nervous system similar to this to model how messages are carried from the senses to the brain. Tutorial.
You could build a wind tube and demonstrate how some materials and shapes fly better than others. Here’s a tutorial on building a wind tube. I talk about it more in our wind and flight lesson plan. If you already own a good fan, this would be a mid-price point project (more than most, less than the light bulbs).