For anyone who has traveled by airplane or driven up a high mountain pass, the experience of a shampoo bottle leaking or a sealed food container bursting open is a frustrating reality. This phenomenon is a direct demonstration of physics in action. Bottles and other sealed items fail at elevation due to a rapid shift in the environment that throws off the delicate balance between the inside and the outside of the container. We can understand this common travel mishap by examining the atmospheric conditions at altitude and the mechanics of trapped gas.
The External Factor: Atmospheric Pressure at High Altitude
Atmospheric pressure is the immense weight of the air column resting above any given point on Earth. At sea level, the pressure pressing down on us is at its highest, averaging about 14.7 pounds per square inch.
As one gains altitude, whether climbing a mountain or ascending in an aircraft, the total number of air molecules overhead significantly decreases. This reduction in air mass results in a corresponding drop in the external pressure surrounding the container. At high altitude, such as the cruising level of a commercial airliner, the external pressure pushing on a bottle is substantially lower than the pressure at which the bottle was originally sealed.
For instance, the cabin pressure in most commercial airplanes is regulated to simulate an altitude of approximately 6,000 to 8,000 feet above sea level. Even in a pressurized cabin, this environment represents a much lower external force compared to the ground-level pressure where the bottle was packed. This environmental change creates a pressure imbalance, causing the container’s contents to expand.
The Internal Mechanism: How Sealed Containers React
A bottle sealed at ground level traps air and gases inside at the same high pressure as the surrounding atmosphere. When that container is transported to a lower-pressure environment, the internal pressure remains high while the external pressure drops significantly. This creates a large pressure differential across the container walls.
The trapped gases, including air pockets in liquids or carbon dioxide in soda, begin to expand dramatically. This expansion is governed by the inverse relationship between the pressure and volume of a gas. With less outside pressure pushing inward, the high-pressure gas inside exerts a greater outward force on the container walls.
In a carbonated beverage, the dissolved carbon dioxide gas escapes the liquid more readily in the low-pressure environment. This rapidly expanding gas pushes against the sides and lid of the bottle. The outward strain continues until the container reaches its structural limit, resulting in a leak through the seal or a rupture of the container.
Flexible containers, like plastic travel bottles, often visibly bulge as the internal gas expands to equalize the pressure differential. Rigid containers, such as glass bottles, are less forgiving and are far more likely to experience structural failure. While temperature changes can also affect gas volume, the large drop in external atmospheric pressure is the primary mechanism forcing the contents to escape.
Preventing Explosions and Leaks While Traveling
The key to preventing leaks and bursts is to minimize the pressure differential before the trip begins. For liquids, the most effective solution is to “pre-vent” the container. This involves opening the bottle, gently squeezing out the excess air trapped inside, and then securely re-sealing it while maintaining the squeeze.
This action adjusts the internal pressure closer to the expected lower altitude pressure, leaving room for remaining gas to expand without straining the container. It is also beneficial to avoid filling containers completely, leaving a small air gap at the top. This empty space provides a buffer, allowing the expanding gas to occupy a larger volume without forcing the liquid out through the seal.
The choice of container material also plays a role in mitigation. Soft-sided plastic travel tubes and bottles are preferable to rigid materials like glass or metal. Flexible walls can accommodate the expanding internal gas by bulging slightly, which prevents the pressure from building to a point of structural failure. For an extra layer of protection, placing a small piece of plastic wrap or foil over the bottle opening before screwing on the cap creates a simple, secondary seal against minor leaks.