Plumes from Using Iron to Boil Liquid Nitrogen to Illustrate the Importance of Surface Area

Abstract

The relationship between surface area and dynamics of processes can be demonstrated by adding iron at room temperature to liquid nitrogen. The rate at which the liquid nitrogen boils to produce gas is related to the surface area of the iron. Adding iron in the form of consistent units that have measurable sizes can be readily connected to observable differences in rates of nitrogen gas production. For example, samples of smaller iron spheres with their greater surface area transfer heat more quickly than do larger spheres of the same volume to liquid nitrogen causing it to boil faster, but more briefly, and produce larger plumes of nitrogen gas from a container vent. The plumes are essentially comprised of nitrogen and water, which make them potentially safer than plumes from other demonstrations such as the "genie in a bottle", based on hydrogen peroxide decomposition. These simple activities can be used as stand-alone demonstrations or as the basis of laboratory activities.

Dealing with Dust Entrained in the Nitrogen Plume Demonstration

“…An alternative to the visual observation of condensation plumes from the vent of the container containing liquid nitrogen is to place an anemometer over the bottle vent . A less sophisticated but more entertaining approach involving a noisemaker is shown in Figure .…”

“…This can be shown by comparing the plumes produced using two samples of iron with the same mass, in which one sample is divided into a few spheres and the other is divided into many smaller particles. The more finely divided samples have larger surface area and will therefore boil the liquid nitrogen more quickly to produce larger condensation plumes . For example, in a previous paper in this Journal , 33 g samples of metallic iron were added to liquid nitrogen in the form of 4 half-inch (12.7 mm) diameter spheres, ∼32 quarter-inch (6.4 mm) diameter spheres, ∼256 eighth-inch (3.2 mm) diameter spheres, or iron powder.…”

“…In this demonstration, we poured liquid nitrogen into the bottom of the milk jug until it was about 1/8 full. The same dropping mechanism from the previously described demonstration was used: a lid from a milk jug with a centrifuge tube pierced through it, as shown in Figure . The tube was filled with about 33 g of iron spheres or powder, and a magnet held the iron in place by placing it outside the tube so the jug lid could be screwed on; then, the magnet was removed to drop the iron into the nitrogen to sound the whistle.…”

“…Water condensation plumes can be produced by adding iron spheres to liquid nitrogen in a vented container such as a plastic milk jug, as described in a recent paper in this Journal. 1 When the spheres at room temperature transfer their energy as heat to liquid nitrogen, the cold nitrogen vapor escapes the vent and condenses water vapor in the surrounding air to make the small liquid water droplets that make the plume visible. In addition to the visual appeal of a plume of fog, these demonstrations illustrate the relationship between surface area and kinetics of processes.…”

“…The more finely divided samples have larger surface area and will therefore boil the liquid nitrogen more quickly to produce larger condensation plumes. 1 For example, in a previous paper in this Journal, 33 g samples of metallic iron were added to liquid nitrogen in the form of 4 half-inch (12.7 mm) diameter spheres, ∼32 quarterinch (6.4 mm) diameter spheres, ∼256 eighth-inch (3.2 mm) diameter spheres, or iron powder. Samples composed of smaller spheres or particles of iron had greater surface area and produced larger plumes more quickly.…”