Liquid nitrogen can be used as a cryogenic substance to obtain low temperatures. Under atmospheric pressure, liquid nitrogen boils at 77 K, allowing low temperatures to be reached. However, if the nitrogen is placed in a sealed, insulated container connected to a vacuum pump, even lower temperatures can be reached. Why? If the vacuum pump has sufficient capacity and is left on for an extended period of time, the liquid nitrogen starts to freeze. Explain.

Liquid nitrogen is a colorless, odorless, inert, and nontoxic cryogenic fluid with the formula N₂. It is capable of producing frigid temperatures as low as 77 K (-196°C or -321°F) under atmospheric conditions. Due to its extremely low temperature, it is commonly used in a variety of applications, including cryopreservation, cryotherapy, and as a coolant for superconductors. It is produced industrially by the fractional distillation of liquid air.

Without being exposed to external heat sources, liquid nitrogen can maintain its low temperature due to its low vapor pressure, which minimizes the rate of evaporation. When handling liquid nitrogen, safety precautions are essential because contact can cause severe frostbite, and the rapid expansion of the liquid to gas can build pressure in containers.

Vapor pressure is a property of a liquid that describes how readily particles escape from the liquid phase to the gas phase. It is dependent on temperature; as a liquid gets warmer, its vapor pressure increases. At any given temperature, a substance will have a specific vapor pressure. When the vapor pressure equals the ambient pressure, the liquid will boil.

The vapor pressure of liquid nitrogen changes with temperature, but since it is generally very low, it does not boil and evaporate as quickly as a liquid with a higher vapor pressure at the same temperature. The control of vapor pressure is critical in processes like distillation and plays a role in why liquid nitrogen can be used to achieve ultra-low temperatures.

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure. For liquid nitrogen, the boiling point under one atmosphere of pressure is 77 K. Boiling occurs when the molecules have enough energy to break the intermolecular forces that hold them in the liquid state and escape into the gas phase.

Understanding the boiling point is crucial when using cryogenics to reach low temperatures, since adjustments in ambient pressure can change the boiling point. This knowledge is applied in methods such as the one outlined in the exercise, where reducing pressure with a vacuum pump lowers the boiling point, allowing temperatures colder than the natural boiling point of nitrogen to be reached.

The heat of vaporization is the amount of energy required to change a substance from a liquid to a gas at its boiling point. This energy is used to overcome the attractive forces within the liquid. For nitrogen, this value is significant, meaning that as nitrogen vaporizes, it absorbs a considerable amount of heat from its surroundings, creating a cooling effect.

In the context of the exercise, the continuous boiling of nitrogen caused by the vacuum pump results in additional absorption of heat from the remaining liquid. This process is an endothermic phase transition, cooling the liquid further and potentially leading to the freezing of the remaining nitrogen if the heat of vaporization continues to be removed efficiently.

The freezing point of nitrogen, the temperature at which it transitions from a liquid to a solid, is 63 K (-210°C or -346°F). Under standard atmospheric pressure, nitrogen generally does not reach these temperatures without artificial cooling techniques.

In the scenario provided in the exercise, the freezing point of nitrogen is achievable because the liquid nitrogen's surrounding pressure is lowered using a vacuum pump, which decreases its boiling point. As the nitrogen boils, it absorbs heat from itself, which in turn lowers the temperature of the remaining liquid until it reaches the freezing point. This principle is widely used in cryogenics to obtain and maintain low-temperature conditions for various scientific and industrial applications.