Elsa made a lava lamp! She used oil, water, food colouring and Alka Seltzer tables. How did this happen? The oil floats on top of the water because it is less dense or lighter than water. The food coloring has the same density as the water so it sink through the oil and mixes with the water. When you add the tablet it sinks to the bottom then starts to dissolve. As it dissolves it makes gas, carbon dioxide.