The creation of a cloud inside a bottle is a classic demonstration that effectively models the complex atmospheric processes responsible for weather phenomena. This simple tabletop experiment allows for the replication of cloud formation by isolating and manipulating the three fundamental ingredients required for water vapor to condense into visible droplets. The goal is to successfully contain and then abruptly change the environment of humid air within a sealed chamber.
Essential Materials and Setup
A successful cloud demonstration requires a few readily available items, starting with a transparent and flexible container. An empty two-liter plastic soda bottle with its cap is ideal because its walls can be compressed to change the internal pressure.
The experiment requires a source of water vapor and airborne particles called condensation nuclei. Warm tap water provides the necessary water vapor through evaporation, saturating the air inside the bottle. For the nuclei, use smoke from an extinguished match or a few drops of isopropyl rubbing alcohol.
If using a match, an adult should manage the flame, dropping the smoking end into the bottle before sealing the cap immediately to trap the particles. Rubbing alcohol is an alternative that eliminates the fire risk and provides a more dramatic cloud effect. Safety goggles are recommended when forcefully compressing the bottle.
Detailed Step-by-Step Instructions
Pour a small amount of warm water into the two-liter plastic bottle, using just enough to cover the base. Swirl the water around for about thirty seconds to increase the humidity inside the bottle by encouraging evaporation.
Next, add the condensation nuclei. An adult should carefully light a wooden match, extinguish the flame, and immediately drop the smoking end into the bottle. The cap must be sealed tightly within a second or two to trap the smoke particles.
With the cap secured, rapidly manipulate the pressure. Squeeze the bottle firmly for several seconds, which slightly increases the air temperature inside. Abruptly release the pressure completely, causing an instantaneous drop in temperature. A cloud should immediately appear following this sudden release. Repeating the squeeze-and-release cycle will make the cloud denser and more visible.
Understanding the Physics of Condensation
The experiment creates a cloud because it fulfills the three requirements for condensation: adequate water vapor, condensation nuclei, and a drop in temperature. The warm water ensures a high concentration of invisible water vapor molecules are suspended in the air inside the sealed bottle. This vapor remains in a gaseous state until forced to change phase.
The smoke particles or alcohol molecules serve as condensation nuclei, which are microscopic surfaces around which the water vapor can cluster. Without these particles, the water vapor molecules would struggle to form visible droplets, a state known as supersaturation. In the atmosphere, these nuclei are provided by dust, pollen, and pollutants.
The final factor is the rapid temperature change, achieved through adiabatic cooling. When the bottle is squeezed, the pressure on the air increases, and the air temperature rises slightly. The sudden release causes the air to rapidly expand back to its original volume. This expansion forces the air molecules to do work against the surrounding pressure, drawing energy from the air itself.
This rapid energy loss causes the temperature inside the bottle to drop suddenly. The cooled air can no longer hold as much water vapor, forcing the vapor to condense onto the awaiting nuclei and forming the visible cloud.