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Ch.12 - Liquids, Solids & Intermolecular Forces
Chapter 12, Problem 75

Nitrogen has a normal boiling point of 77.3 K and a melting point (at 1 atm) of 63.1 K. Its critical temperature is 126.2 K and its critical pressure is 2.55×104 torr. It has a triple point at 63.1 K and 94.0 torr. Sketch the phase diagram for nitrogen. Does nitrogen have a stable liquid state at 1 atm?

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Phase Diagram

A phase diagram is a graphical representation that shows the states of a substance (solid, liquid, gas) at various temperatures and pressures. It includes key features such as the lines of equilibrium between phases, the critical point, and the triple point, where all three phases coexist. Understanding how to read and interpret a phase diagram is essential for analyzing the behavior of substances under different conditions.
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Triple Point

The triple point of a substance is the specific temperature and pressure at which all three phases (solid, liquid, and gas) can coexist in equilibrium. For nitrogen, this occurs at 63.1 K and 94.0 torr. This concept is crucial for understanding the conditions under which a substance can exist as a liquid, and it helps determine the stability of the liquid phase at various pressures.
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Critical Temperature and Pressure

The critical temperature is the highest temperature at which a substance can exist as a liquid, regardless of pressure, while the critical pressure is the minimum pressure required to maintain the liquid state at this temperature. For nitrogen, the critical temperature is 126.2 K and the critical pressure is 2.55 * 10^4 torr. These concepts are vital for determining whether nitrogen can exist as a stable liquid at 1 atm, which is below its critical temperature.
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