Why do you hear a glugging sound when you pour a liquid out of a bottle by holding it upside down?Asked by: Matt
AnswerThat is just air rushing in to fill the space left above the liquid. As the liquid leaves, the pressure above it decreases until the larger atmospheric pressure outside the bottle forces air in to equalize pressure.
You'll notice that if you pour out the liquid gently, leaving an open space between it and the top of the bottle opening, air can enter more gradually, without the 'glugging'.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor