First you'll need to make a super saturated salt solution (a.k.a. brine) by boiling 2cups of water and then adding salt in batches, stirring in between, until the salt no longer dissolves. Let it cool.
Water (hot and cold)
3 small glass containers
Put equal amounts of brine, ice water, and hot tap water into each of the three containers (we used special plastic test tubes, the ones from which 2L bottles are made, with 2 tablespoons of liquid in each.)
Add a drop of food coloring into each at approximately the same time and watch what happens. In this video, red is hot, yellow/green is cold, and blue is brine.
The red and the yellow/green drops sank at about the same rate, but the red diffused throughout the water much faster. You can see this better with purple dye better (purple is cold, red is hot water.)
Density is mass (g) per volume (ml) or
D = m/vFor a liquid this is easy to calculate. For a solid you can figure out the volume by how much water it displaces (see pg. 17 of the encyclopedia to set this up.)
Chem1 Virtual Textbook: Density and Buoyancy (advanced)
I Love Density has a complete science project on density (intermediate)
We found several Gizmos that demonstrated density. One had objects on a shelf that you could put on a scale to get weight (mass), then in a graduated cylinder to get volume. I did the calculation for the kids (they were fractions that I converted to a decimal.) You can then put the object in a liquid in which you can adjust the density. The kids were impressed when I started out with the object floating and as soon as I adjusted the fluid density to a number less than the object, the object sank.