Have you ever noticed the gulping sound and rising bubbles when you empty a water bottle by holding it vertically downward? Have you ever wondered why this happens?
Before emptying the water bottle, the atmospheric pressure inside and outside the bottles remains the same.
However as you tilt the bottle downward, the water starts to flow out through the nozzle.
When some water leave from the bottle, the air expands inside the bottle causing the air pressure inside the bottle to decrease.
Due to higher pressure of air outside, the air forces its way back into the bottle.
The gulping sound you hear is due to the air being sucked in the bottle and temporarily halting the flow at the bottle opening. This is how an air bubble is create at the opening of the bottle which is then moved in the upward direction through the water and popped at the water surface at the top. This equalizes the pressure in the bottle and the water then flows again. Then the pressure drops again, air again gets sucked in thereby repeating the process described above.
As the process repeats, the pressure drops again, drawing in more air, forming more bubbles, and continuing the cycle. Eventually, all the water is emptied, and the bottle is filled with air at atmospheric pressure.
Next time, when you will empty the water bottle, observe how air bubbles are being created as a result of the pressure imbalance inside and outside of the bottle.