My son has chosen a science fair project wanting to test the affect of water containing different substances (tap, salt, baking soda, etc) on voltage output. I'm too ignorant on elctrical current flows (other than water and electricity don't play well together) to advise him. Can anyone give me some advice on how we could possibly set this up or should he scrap the idea before I end up on the top ten list of how to improve the human gene pool?

Thanks,

Eagle-Driver

2 ways come to mind. Both include using 2 pieces of #12(20 amp electrical wire)Solid, or romex? Attach to container, with both having 6" of wire in solution, try all three.
You could use an Ohm meter(Multimeter on ohm scale)and check the resistance of each solution. Start on Highest ohm scale. You will be measuring Resistance. And current will be flowing through solution.
Another way, would be a battery, go through the water and back out. Now you will be measuring voltage.

One other way is to choose two different electrodes and effectively make a battery with the solution as an electrolyte. If memory serves me well you can make a battery with a lemon and zinc electrode. The chemistry of this is long forgotten.

This shows what I believe you are working on:
Water conductivity - Science at Home