Answer
Hot-air balloons float because the gas molecules in hot air balloon move faster as the volume increases and thus this increase in temperature and volume allows balloons to float. Buoyant force is due to the air that is displaced by the balloons and since the weight of air that is displaced in this case is lower and the hot air balloon will float upwards.
Work Step by Step
The Charle's gas law defines the relationship between temperature and volume.
According to this, the volume of gas is directly proportional to its temperature at a constant pressure.
$\frac{V}{T}$ = constant
Hot-air balloons float because the gas molecules in hot air balloon move faster as the volume increases and thus this increase in temperature and volume allows balloons to float. Buoyant force is due to the air that is displaced by the balloons and since the weight of air that is displaced in this case is lower and the hot air balloon will float upwards.
