Why does water boil cause the circular motion?

The correct answer to this question is Convection. This is the movement that happens in a fluid. The movement causes cold and dense material to sink, as well as makes hot and dense material rise. The boiling of water is the most common example of convection. When water boils, heat passes to the pot from the burner.

This passage heats the water at the bottom of the pan. Once the water at the bottom is hot, it rises and in exchange, the water that is cooler moves to the bottom. Other examples of convection include a hot air balloon and the melting of ice.

Convection occurs when water is heated to boiling. Fluids expand when heated, and the molecules move faster bouncing against one another, and this causes the water to circulate to the top to cool and then back to the bottom to replace the heated water rising. The movement produced within a fluid by the tendency of hotter and therefore becomes less dense and it will rise and in turn the cold-water sinks.

This is the transfer of thermal energy by the actual physical movement of warm air. If the heat is still supplied the cooler fluid that flows in to replace the rising warmer liquid will also become heated and rise. A convection current becomes established in the fluid with warmer, less dense fluid rising from the point of application of heat and cooler, more dense portions of fluid sinking.