What occurs to the portion of liquid that first begins to warm on a burner?

When a portion of liquid is heated on a burner, the first part to warm up experiences an increase in temperature, causing its molecules to gain kinetic energy. As the temperature rises, the liquid's density decreases. This reduction in density means that the warmer liquid will rise upward through the cooler, denser liquid surrounding it.

This upward movement is a fundamental aspect of convection, where warm fluids rise, and cooler fluids sink. It creates a convection current as the process repeats—warm liquid rises, cools, and then sinks back down into the heat source. This cyclical pattern is essential for the mixing and even heating of liquids.

In contrast, the other options describe behaviors inconsistent with the principles of how heated liquids behave. For instance, an increase in thermal energy would not lead to greater density, and a cooler, denser liquid would sink, not the heated portion.