Introduce density using raisins and soda. Pour clear soda into a container and add some raisins. As the raisins, which have a higher density than the soda, sink to the bottom, bubbles cling to their sides, and eventually cause them to rise. When they reach the surface, the bubbles burst and they sink again. With the bubbles of gas clinging to them, the raisins become lower in density than the soda, and they float. For a variation, try heating the soda. Warm water has a different density than cold, which can give students a fun way to experiment with different variables.
Demonstrates how the density of a gas is different than its mass. Fill a mylar balloon with helium just enough to get it to float. Tie a ribbon to the balloon and trim it until the balloon hangs in space, neither floating up, nor sinking. Blow hot air on the balloon using a hair dryer. The sides will expand and the balloon rises. When the air cools, the balloon drops. The heat doesn't change the mass. The balloon expands because the moving molecules force the sides out. It has become less dense, not lighter. As the air cools, the molecules slow down, making it more dense, not heavier.
Explore liquid density using salt water. Fill five jugs with one gallon of water each. In the first, add 1/2 cup pickling salt. Mix one cup of salt into the second, 1 1/2 cups into the third, and two cups into the fourth. Leave the last as plain water. Color each solution with a different shade of food coloring. Give students five test tubes containing each solution. Draw some of the first solution, using either a straw or an eye dropper, and add it to the test tube of the second. If the solution floats, it is less dense. If it mixes or sinks, it is less dense. Continue experimenting with each solution, and ask students to establish the order of density.
Show students that liquids can share the same volume, but have different densities. Pour eight ounces each of honey, corn syrup, dish soap, water, vegetable oil, rubbing alcohol, and lamp oil into separate cups. Color all but the honey and vegetable oil with different shades of food coloring. Pour the honey into a cylindrical container. Slowly pour the corn syrup on top. Follow with dish soap, water, vegetable oil, alcohol, and then the lamp oil. When finished, you see separate liquids layered on top of each other. The same amount of two liquids will have different weights because they have different densities.