How Everything Works
How Everything Works How Everything Works

Question 502

What are the two substances in a Lava Lamp, and why do they react the way they do?
I'm afraid that I'm unable to determine exactly what substances the lamp contains. However, I believe that one of them is water and the other is a high-density wax. When the lamp is cold, the wax is a crystal solid with a density slightly higher than that of water. Because the buoyant force this wax experiences from the water is less than its weight, the wax sinks to the bottom of the lamp. But when the lamp is on, the bottom of its container heats up and the wax begins to melt. Like most materials, wax's liquid phase is substantially less dense than its solid phase. As it melts, the wax expands so much that its density drops below the density of water and it floats upward to the cool top of the container. Once it reaches the top, the wax begins to solidify. As it solidifies, the wax contracts so much that its density rises above the density of water and it sinks downward to the bottom of the container. Thus when the lamp is in full operation, the rising bubbles of wax are liquid and the descending bubbles of wax are solid. Dyes are added to the two materials to make them more visible—the water is colored by a water-soluble dye (perhaps food coloring) while the wax is colored by an oil-soluble dye (like those used in permanent markers).

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