What physical change occurs to the molecules in a lava lamp as they are heated at the base?

In a lava lamp, when the molecules at the base are heated, they gain energy. This increase in energy causes the molecules to move faster and spread apart, leading to a decrease in their density. As these heated, less dense molecules rise through the cooler liquid above, they create the iconic flow and movement seen in a lava lamp. The process is a demonstration of convection, where warmer, less dense fluids rise and cooler, denser fluids sink.

The other answers do not accurately describe the behavior of the molecules in this scenario. The molecules do not lose energy and sink or change into a solid form; rather, they transition into a less dense state as they are heated. The reference to expansion and becoming less dense at the top is somewhat correct in context, but it lacks the precise mechanism of gaining energy and rising, which is essential to understanding convection.