Final answer:
The upward motion of air in flames is a result of buoyancy, driven by the lower density of hot air as explained by the ideal gas law, causing the hot air to rise.
Step-by-step explanation:
The upward motion of air in flames can be explained by both the ideal gas law and the concept of buoyancy. According to the ideal gas law (PV=nRT), as the temperature of a gas increases, its volume increases if the pressure and number of moles remain constant.
This decrease in density in comparison to the cooler surrounding air leads to buoyancy, and thus, the hot air rises. In the context of a flame, the air is heated by combustion, which reduces its density, resulting in an upward motion due to the buoyant force.