The “Harbottle” is a glass bottle with a hole in the bottom that can be plugged with a stopper. When a balloon is inflated inside the Harbottle and the hole stopped up, the balloon stays inflated!

Materials:

• Glass Harbottle [Cabinet D3]
• Balloon
• Rubber stopper
• Water source (optional)

Procedure:

• Remove the rubber stopper from the Harbottle.
• Place the balloon inside the Harbottle and stretch the mouth around the Harbottle’s neck to ensure a tight seal.
• Inflate the balloon. While keeping the balloon inflated, put the rubber stopper into the hole on the bottom of the Harbottle.
• The balloon will stay inflated!

Water demo:

• Inflate the balloon in the Harbottle and plug in the stopper (as in the Procedure above).
• Fill the balloon with water.
• Remove the stopper: water will shoot out of the balloon!

Explanation:
When a balloon is inflated, the air pressure inside the balloon (P_B) is higher than the air pressure outside (P_Air). The inside air pressure pushing the balloon skin outward exactly counteracts the elastic force in the balloon skin that pulls it inward. Due to the pressure difference, however, if an inflated balloon is re-opened to the outside air, the air inside will rush out such that the pressures in and out of the balloon will equalize.

Inside the Harbottle, we start out with the same situation as before:

 

 

 

 

 

 

When the balloon is uninflated, the air pressures outside, in the balloon, and in the Harbottle (P_H) are all equivalent (left image). When the balloon is initially inflated and sealed off at the mouth, the air pressure inside the balloon is higher than that in the Harbottle and in the surrounding air: the balloon will stay inflated (right image).

Once the stopper is put into the Harbottle, however, a non-intuitive thing happens: the balloon inside stays inflated! The key to what is going on is that even though the air pressure inside the balloon is the same as that outside the Harbottle, P_B  is lower than the pressure inside the Harbottle. A balloon will stay inflated if the pressure inside is higher than that outside.

The question: Why is the pressure inside the Harbottle lower than the pressure in the balloon and the outside air pressure?

The answer: Once the stopper is placed into the Harbottle, air can no longer enter or escape. When the balloon is opened up to the outside air pressure, the pressure inside the balloon and the air pressure equalize. The balloon deflates a bit during this process (and shrinks, as indicated with the arrows in the image above). As the balloon shrinks, the volume of air inside the Harbottle increases, and consequently, the pressure P_H decreases.

This effect is visible if you watch the balloon carefully after you put in the stopper and unseal the balloon – it will shrink very slightly! (If you only inflate the balloon a little bit, the effect is more pronounced).