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In class we watched a video on how lava lamps work, explain why the wax floats and sinks in the lamp.
Typically wax has a higher density then water so it sinks, however in a lava lamp there is a bulb at the bottom that heats the wax up and causes the molecules in the wax to expand which increases its volume. When the volume of the wax increases the density decreases which causes it to float.