Glass container Vegetable oil Water Food coloring Alka-Seltzer tablet Oil and water do not mix. When you pour the water into the bottle with the oil, the water sinks to the bottom and the oil floats to the top. Oil floats on the surface because water is heavier than oil. Water is more dense than the oil. The Alka-Seltzer tablet reacts with the water to make carbon dioxide gas. These bubbles attach themselves to the colored water and cause them to float to the surface. When the bubbles pop, the color sinks back to the bottom of the glass.