Final answer:
Bubbling occurs in the membrane-type humidifier due to the evaporation and condensation of water vapor.
Step-by-step explanation:
Bubbling does occur in the membrane-type humidifier. This happens because the humidifier consists of a membrane that allows water to evaporate and become water vapor. The water vapor then condenses back into liquid form, creating bubbles. These bubbles help to humidify the air.