A phase diagram plots pressure against temperature, defining the precise conditions under which a material exists as a solid, liquid, or gas. Understanding the specific phase diagram for nitrogen (\(\text{N}_2\)) is necessary for predicting its behavior across a wide range of environments. Nitrogen makes up about 78% of Earth’s atmosphere and is a diatomic molecule. The diagram helps engineers and scientists to safely manipulate nitrogen’s state for its many industrial and technological uses.
Reading the Pressure and Temperature Axes
A typical phase diagram uses the vertical axis to represent pressure and the horizontal axis to represent temperature. Any single point on this two-dimensional graph corresponds to a unique combination of pressure and temperature for the substance. The units are commonly expressed in atmospheres (atm) or megapascals (MPa) for pressure and Kelvin (K) or Celsius (\(\text{^\circ}\)C) for temperature.
The regions on the diagram indicate the conditions where a single phase—solid, liquid, or gas—is stable. The curved lines, or phase boundaries, represent conditions where two phases can coexist in equilibrium. Crossing any of these boundary lines indicates a phase transition, such as melting, freezing, boiling, or sublimation.
The Primary Phases of Nitrogen
The diagram is divided into three major areas, each corresponding to a distinct phase of nitrogen. The gas phase occupies the largest area on the diagram, found at high temperatures and low pressures, which is the state nitrogen exists in naturally as a component of air. Gaseous \(\text{N}_2\) molecules are widely separated and move freely, filling any container they occupy.
The liquid phase of nitrogen, known as liquid nitrogen (\(\text{LN}_2\)), is stable within a low-temperature range. It has a boiling point of \(77.35\text{ K}\) (\(\approx -195.8\text{^\circ}\text{C}\)) at standard atmospheric pressure. Liquid nitrogen is a transparent, non-toxic fluid where molecules are close together but still able to move past one another.
The solid phase is located on the far left of the diagram, corresponding to the lowest temperatures. Solid nitrogen is formed when the liquid is cooled further, with a melting point of \(63.29\text{ K}\) (\(\approx -209.86\text{^\circ}\text{C}\)) at one atmosphere of pressure. In this state, the \(\text{N}_2\) molecules are tightly packed in a regular structure, with only vibrational movement occurring.
Defining the Triple and Critical Points
The triple point is the combination of temperature and pressure at which solid, liquid, and gaseous nitrogen can all coexist in thermodynamic equilibrium. For nitrogen, this occurs at approximately \(63.15\text{ K}\) (\(\approx -210\text{^\circ}\text{C}\)) and a pressure of \(0.124\text{ atm}\). At any condition below this triple point pressure, liquid nitrogen cannot exist, and heating the solid will cause it to sublime directly into a gas.
The critical point is the highest temperature and pressure at which nitrogen can exist as a distinct liquid and gas. Beyond this point, the liquid and gas phases become indistinguishable, forming a single state of matter called a supercritical fluid. Nitrogen’s critical point is defined by a critical temperature of \(126.2\text{ K}\) (\(\approx -147\text{^\circ}\text{C}\)) and a critical pressure of \(33.55\text{ atm}\) (\(\approx 3.4\text{ MPa}\)). A supercritical fluid possesses properties between those of a gas and a liquid, exhibiting gas-like diffusivity and liquid-like density.
Industrial Applications of the Diagram
Knowledge of the nitrogen phase diagram is essential for managing industrial processes that involve temperature and pressure control. For cryogenics, the diagram confirms that nitrogen must be maintained below its boiling point of \(77.35\text{ K}\) to remain liquid for applications like preserving biological samples or freezing food. The diagram guides the design of storage tanks and transfer systems to prevent premature vaporization.
In the chemical industry, the phase boundaries are used to optimize the process of air distillation, which separates nitrogen from oxygen. By precisely controlling the temperature and pressure, engineers can ensure nitrogen liquefies efficiently for collection.
Furthermore, the diagram is consulted in high-pressure applications, such as inerting systems in chemical plants and oil and gas operations. Using nitrogen’s phase data allows operators to maintain a safe, oxygen-free blanket over volatile materials, preventing explosions and fires.