The secret of the “Balloon in the bottle” trick

An awe­some ex­per­i­ment that will make you a su­per­star at any par­ty!

Safe­ty pre­cau­tions

None.

Equip­ment

• rec­tan­gu­lar plas­tic bot­tle;
• bal­loon.

Step-by-step in­struc­tions

In­sert a bal­loon into a rec­tan­gu­lar plas­tic bot­tle and se­cure it around the bot­tle neck. We rec­om­mend us­ing a rec­tan­gu­lar bot­tle be­cause it is more sta­ble and will hold its shape bet­ter. Try to in­flate the bal­loon – you can’t. Just make an in­con­spic­u­ous hole in the bot­tle and try again – the bal­loon should in­flate!

Process de­scrip­tion

In­stinc­tive­ly, we’d prob­a­bly say the bot­tle is emp­ty, but it isn’t – it’s filled with air. In in­sert­ing a bal­loon into the bot­tle, we cre­ate a closed sys­tem, the pres­sure in which is equal to at­mo­spher­ic. In this closed sys­tem, our lungs can’t ap­ply the pres­sure need­ed to com­press the air in­side the bot­tle enough to free the space for the bal­loon. The trick is sim­ply to make an in­con­spic­u­ous hole in the bot­tom of the bot­tle. This al­lows the ex­cess air to leave the bot­tle, which lets you in­flate the bal­loon. The hole must then be plugged with a fin­ger or taped to pre­vent the bal­loon from de­flat­ing.

The bal­loon de­flates slight­ly at the mo­ment we close the hole, marginal­ly de­creas­ing the air pres­sure in the bot­tle. This dif­fer­ence be­tween the at­mo­spher­ic pres­sure in­side the bal­loon and the re­duced pres­sure in the bot­tle keeps the bal­loon in­flat­ed. The bal­loon’s de­fla­tion is prac­ti­cal­ly im­per­cep­ti­ble – only a minis­cule change in the vol­ume of free space in the bot­tle is nec­es­sary for the bal­loon to stay in­flat­ed. When we let air in through the tiny hole, the pres­sure in the bot­tle grad­u­al­ly equal­izes with at­mo­spher­ic pres­sure, and the bal­loon de­flates due to its own elas­tic­i­ty.