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.
When a laser of sufficient intensity is shined into a pigmented transparent medium, the partially absorbed laser light can heat and change the refractive index of the medium in the path of the beam. This altered medium will in turn cause the transmitted laser light to thermally defocus into a series of concentric rings. Some solvents such as 2-propanol and a variety of polymers such as polystyrene, polyethylene terephthalate, and polydimethylsiloxane can be pigmented with a variety of colors. Samples of various colors can be used to spectacularly demonstrate characteristics of light absorption. For example, a 2-propanol solution containing methyl red in red acidic form will absorb a green laser and produce interference rings, but a solution of methyl red in yellow basic form will not. As another example, a red laser beam will pass unaffected through a red pigmented film, but the beam will heat a green film (e.g., from a green pigmented polyethylene terephthalate soda bottle) to produce interference rings. Additionally, since the pattern produced is based on the interference of light waves, these demonstrations reinforce concepts associated with the wave nature of light.
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