The can crush demonstration is an experiment that offers an immediate visual explanation of fundamental concepts in physics. It involves manipulating an ordinary aluminum beverage container to make it spontaneously collapse without being touched by human hands. The spectacle of the can crumpling inward seems almost magical, but it is entirely based on the principles governing gases and forces. Understanding the science behind this implosion requires a closer look at how internal conditions are rapidly changed and how external forces are allowed to dominate.
Setting Up the Demonstration
The experiment requires an empty aluminum can, a small amount of water, a heat source, tongs, and a large container of cold water, preferably with ice. Place approximately 10 to 15 milliliters of water into the can and set it on the heat source. As the water boils, watch for steam continuously escaping the can’s opening, which signals the preparation is complete. Protective measures are necessary, including wearing safety eyewear and using heat-resistant tongs to manipulate the hot can.
Creating the Internal Vacuum
Heating the small amount of water inside the can causes a phase change from liquid to gas, generating steam. This steam rapidly expands and fills the entire volume of the can, pushing the air molecules out through the opening. After a minute or two of vigorous boiling, the can’s internal atmosphere consists almost entirely of high-temperature water vapor.
The Crushing Force
The collapse is triggered when the hot can, full of steam, is quickly inverted and plunged into the cold water, sealing the opening. The cold water causes the high-temperature water vapor inside to instantly condense back into a few drops of liquid water. Since liquid water occupies a volume thousands of times smaller than steam, this rapid condensation creates a near-vacuum inside the can. This internal pressure drop allows the force of the air surrounding the can to act without opposition. Atmospheric pressure presses on every surface with an average magnitude of about 14.7 pounds per square inch (psi) at sea level. This external force, normally balanced by the air inside, is now unopposed and pushes inward on the aluminum walls, causing the collapse.
Pressure at Work in Everyday Life
The principle of pressure differential that crushes the can is constantly at work in various everyday scenarios. When drinking through a straw, removing air lowers the pressure inside the tube. The higher atmospheric pressure on the liquid outside then pushes the liquid up into the low-pressure area. A vacuum cleaner operates similarly, using a fan to create a low-pressure region inside the device, causing surrounding high-pressure air to rush in, carrying dust and debris. Even the weather is governed by pressure differences, as wind is air moving from high-pressure areas to low-pressure areas to achieve balance.