A common experience when traveling to a mountain destination or flying on an airplane is noticing how snack bags appear taut and dramatically inflated. This striking change from a standard, flexible package to a balloon-like vessel often sparks curiosity about the contents. The observation of a puffed-up chip bag is not a packaging defect or a marketing trick, but a clear demonstration of fundamental atmospheric physics in action. This phenomenon is a direct result of the difference between the air pressure inside the sealed bag and the air pressure of the surrounding environment.
Understanding Atmospheric Pressure
Atmospheric pressure is essentially the weight of the air column pressing down on a surface. This pressure exists because air, though invisible, is composed of countless molecules that are pulled toward the Earth by gravity. At sea level, a person has the entire atmosphere’s mass pressing down on them, which is approximately 14.7 pounds per square inch (psi).
As altitude increases, the air becomes less dense because there are fewer air molecules remaining above that point. This reduction in the overlying mass of air directly translates to a decrease in atmospheric pressure. For instance, at an elevation of about 10,000 feet, the external pressure can be nearly 30% lower than the pressure at sea level. This external environmental factor is the key condition that drives the inflation of a sealed package.
How Chip Bags Are Sealed
The contents of a snack bag are sealed under specific pressure conditions, which establishes the internal environment. Most commercial food packaging operations are located at or near sea level, where the atmospheric pressure is at its maximum. This means the air or gas inside the bag is sealed at this higher, sea-level pressure.
The air inside the flexible package is not simply regular air, but is often flushed with an inert gas, such as nitrogen, before sealing. This process, known as modified atmosphere packaging, helps to preserve the chips by preventing oxidation, which keeps them fresh and prevents crushing.
The pressure of the nitrogen gas inside the bag at the moment of sealing is equal to the surrounding atmospheric pressure, creating a balanced state. Once the bag is sealed, this internal pressure becomes a fixed baseline that will not change unless the bag is opened. This internal pressure is the necessary force that will interact with any change in the external environment.
The Science of Expansion
The dramatic inflation of the chip bag occurs because of the pressure differential created by the change in altitude. When the sealed bag is transported to a higher elevation, the external atmospheric pressure begins to drop significantly. However, the pressure of the gas trapped inside the bag remains at the higher, sea-level pressure at which it was sealed.
This creates a state of imbalance where the fixed internal pressure is now substantially greater than the reduced external pressure. The gas molecules inside the bag now exert a stronger outward force than the air molecules outside are pushing inward. This pressure difference forces the flexible walls of the bag outward, increasing the bag’s volume.
The expansion continues until the pressure inside the bag is nearly equal to the new, lower pressure outside, or until the packaging material is stretched to its maximum limit. In extreme cases, the stress on the plastic film can exceed its strength, causing the bag to burst. This expansion is a direct physical consequence of the gas volume increasing in response to the surrounding pressure dropping, which visually demonstrates the power of a simple pressure imbalance.