Could someone suggest an inexpensive substitute for alcohol to be added to water in a float/sink separation tank to lower the density? Thank you.
I don't know about prices or availability. But a good starting point is the linked tables of liquid densities. The field of ionic liquids may also offer some unique alternatives for solutions for float/sink separation systems.
Specific Gravity Of Liquids Table
Why not use solutions with density greater than one? Make solutions of Kosher salt in water of various concentrations, up to near saturation. A little food coloring of the various concentrations can help to distinguish them, and students can be challenged to "stack" the various colors in a transparent or translucent soda straw. This takes some dexterity because the solutions are miscible, but it is an active learning challenge. [Kosher salt makes clear solutions, whereas table salt does not, because of a polymer coating to prevent clumping in a shaker.]