Making a lava lamp is a good elementary school science project. During this project, a student explores the relationships between an acid and a base and between oil and water.
To make a lava lamp, fill three-quarters of a 16-ounce plastic bottle with vegetable oil. Fill the rest of the bottle with water, but leave a little space at the top. Add 10 or more drops of food coloring of your choice, make sure that the color of the water is rich. Break one Alka-Seltzer tablet into six to eight pieces, and add the pieces to the bottle one piece at a time. When the liquid stops bubbling, place the cap back on the bottle, and shake it in different directions. Record your observations.
Water and oil don't mix because they don't form any chemical bonds with each other. This happens because oil consists of long chains of carbon. These chains are hydrophobic, which means that they are repelled by water. The oil floats on top of the water because it is less dense than water.
Alka-Seltzer contains both an acid and a base. The two compounds react when they are mixed with water, producing carbon dioxide as a result. The bubbles within the colored fluid consist of carbon dioxide.