Background+Chemistry

Liquid nitrogen is nitrogen cooled to below 196ºC, so that it is in its liquid state. It is extremely effective at cooling other objects, not only due to its low temperature but also due to the fact is a liquid. A liquid is in contact with a solid to be cooled over its entire surface area. A solid of similar temperature will be much less effective at cooling an object due to the fact that it will have a significantly smaller area of thermal contact with the object to be cooled. This is why dry ice, although very cold, is rather ineffective at freezing many objects.

To make dry ice a more effective cooling substance, it must be made into a liquid form. Unfortunately for this experiment, carbon dioxide does not have a liquid form at standard atmospheric temperatures. It sublimates directly into a gas from its solid form. It can be forced into a liquid at the very large pressure of 56 atm at 20ºC. This would therefore still be ineffective for cooling purposes, as its temperature is much too high to freeze objects. However if the dry ice was to cool a liquid, it would be as effective as a liquid cooling a solid. It is therefore able to quickly cool a liquid. A liquid at near the temperature of the dry ice would be very effective at cooling objects, as it would have excellent thermal contact and a very low temperature.

Most common liquids freeze at temperatures well above that of dry ice, so to cool a liquid to dry ice temperatures would freeze most of them. Isopropyl alcohol however has a very low freezing point, so when used in pure enough concentrations, it can form a sufficiently cold liquid to perform most experiments. It has a very low freezing point in relation to water, because water (H2O) can form hydrogen bonds with all of its atoms. Water is also a bent molecule, making it polar, so there are dipole-dipole forces that keep molecules together. This means it requires a lot of energy in the moving particles, or a high temperature to break it into a liquid. An alkane, on the other end of the scale, is a gas even at temperatures as low as -80ºC. This is because it has no hydrogen bonds or polarity, so it is very easy to break the bonds between molecules apart. Isopropyl alcohol falls between these two, as it can form hydrogen bonds, but on a smaller proportion of its size than water. The carbon atoms allow the molecules to break apart much more easily, making it a liquid at temperatures above -89ºC.

Temperature is a measure of the speed of moving particles in a substance, or more specifically the measurement of the kinetic energy of these particles. The faster the particles of a substance are moving, the higher the temperature. For particles to be moving faster they generally require more space. This is why as a substance gets warmer it changes from solid to liquid then to gas, usually expanding in volume as these changes take place. This change is also due to the kinetic energy of the particles, as it becomes great enough to overcome the strength of the bonds holding molecules close together, allowing them to spread out.

Temperature is spread through substances as the fast moving particles in the one substance collide with particles from another substance. Although no chemical reaction occurs, as the bonds formed already make up the most stable molecules from the given atoms, or the collisions don't have sufficient energy to break the bonds. The collisions do however transfer some of the kinetic energy from the higher temperature substance's particles to those of the smaller substance. This means the higher temperature substance will get cooler and the lower temperature substance gets warmer.

While this dry ice and isopropyl alcohol emulates dry ice for many demonstrations, it is not truly a substitute for liquid nitrogen. Liquid nitrogen is a cryogenic liquid that has many applications in the real world. It is used to freeze cells at low temperature for work in biological applications, as it is able to quickly bring them down to a much colder temperature, where they can stay indefinitely. When thawed, the cells can be used as if fresh, perhaps much later than they would have lived if not cryogenically preserved. It is also used to freeze off skin conditions such as warts. Another one of its uses is to cryogenically freeze dead people or pets, in the hopes that future technology will be able to cure their diseases or bring them back to life.

Liquid nitrogen is especially useful in these situations because it is extremely cold and is a liquid so it makes excellent thermal contact with what it is trying to cool. It is also relatively easy to produce, and fairly safe to handle. The only downfall of it as a cooling substance is due to the Leidenfrost effect, it immediately forms a gas on contact with a much warmer object, which insulates that object and reduces the effectiveness of cooling. This can be counteracted however by instead of using pure liquid nitrogen using a combination of solid and liquid nitrogen, stopping the Leidenfrost effect and still making strong thermal contact.

Therefore, through much research I have determined the most important reasons this "Poor Man's Liquid Nitrogen" is effective are the improved thermal contact of a liquid over a solid, and the low melting points of alcohols, particularly isopropyl alcohol.