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Ch.12 - Liquids, Solids & Intermolecular Forces
Chapter 12, Problem 76

Argon has a normal boiling point of 87.2 K and a melting point (at 1 atm) of 84.1 K. Its critical temperature is 150.8 K and its critical pressure is 48.3 atm. It has a triple point at 83.7 K and 0.68 atm. Sketch the phase diagram for argon. Which has the greater density, solid argon or liquid argon?

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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 points such as the melting point, boiling point, critical point, and triple point, which indicate the conditions under which different phases coexist. Understanding how to read and interpret a phase diagram is essential for analyzing the behavior of substances under varying conditions.
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Density of Phases

Density is defined as mass per unit volume and varies between different phases of a substance. Generally, solids are denser than liquids due to their closely packed molecular structure. In the case of argon, comparing the densities of solid and liquid phases involves understanding how temperature and pressure affect molecular arrangement and spacing, which is crucial for determining which phase has greater density.
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Triple Point

The triple point of a substance is the unique set of conditions at which all three phases (solid, liquid, and gas) coexist in equilibrium. For argon, the triple point occurs at 83.7 K and 0.68 atm, indicating the specific temperature and pressure where solid, liquid, and gaseous argon can exist simultaneously. This concept is vital for understanding phase transitions and the stability of different states of matter.
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