Fog Formation Upon Dry Ice / Hot Water Contact
Pellets of dry ice, solid carbon dioxide, are dumped into a basin of nearly boiling water. A dense white cloud of fog first rises above the basin. As more fog is produced, it stops rising and flows over the rim of the basin and down to the table top.
Illustrates changes in state, density of a heavier than air gas, and how fog forms meteorogically.
Fog forms when water vapor in the air condenses into tiny suspended droplets. This condensation occurs when warm, humid air is cooled. The warm air over the hot water is nearly saturated with water vapor. This warm air is cooled by mixing it with the cold carbon dioxide gas that sublimes from dry ice. Initially the hot water heats the air above it making it less dense and causing the fog to rise. Eventually, the cold carbon dioxide cools the air to the point that it becomes more dense than the air around the basin, and the fog sinks.
Prepare ca. 3 L nearly boiling water and a large quantity (ca. 100-500 g) of dry ice pellets. Handle dry ice with insulated gloves and store in an appropriate insulated container.
Wearing gloves, drop ca. 100-500 g of dry ice into a plastic basin containing ca. 3 L of nearly boiling water (hotter the better). Upon initial contact with the water, an eruption of fog results, followed by the formation of a blanket of fog cascading over the lab bench top and spreading on the floor in all directions. Explain that this is how film makers create fog in the movies, and how fog forms meteorologically.
Solid CO2, dry ice, has a temperature of -78.5 °C and can cause frostbite. Thermal protection in the form of gloves or a towel should be used when handling dry ice.
Solid CO2 should be allowed to sublime in a hood or other well-ventilated area. Water can be disposed of down the drain.