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20. Heat and Temperature

Phase Diagrams, Triple Points and Critical Points

Physics

20. Heat and Temperature

Phase Diagrams, Triple Points and Critical Points

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2:07 minutes

Problem 25a

Textbook Question

At atmospheric pressure, in what phases can CO₂ exist? Refer to Fig. 18 9 6.

Verified step by step guidance

Refer to the phase diagram of CO₂ (Fig. 18.9.6) to understand the relationship between pressure, temperature, and the phases of CO₂. A phase diagram typically shows regions corresponding to solid, liquid, and gas phases, separated by boundary lines.

Identify the atmospheric pressure on the phase diagram. Atmospheric pressure is approximately 1 atm (101.3 kPa). Locate this pressure on the vertical axis of the diagram.

Examine the temperature range at 1 atm pressure. The phase diagram will indicate which phases (solid, liquid, or gas) are present at different temperatures along this pressure line.

Note the triple point of CO₂ on the diagram. The triple point is the unique set of conditions (specific pressure and temperature) where all three phases (solid, liquid, and gas) coexist. For CO₂, the triple point occurs at a pressure higher than 1 atm, meaning liquid CO₂ cannot exist at atmospheric pressure.

Conclude that at atmospheric pressure, CO₂ can only exist in the solid phase (below the sublimation temperature) or the gaseous phase (above the sublimation temperature). Liquid CO₂ is not possible at this pressure.

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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 different phases of a substance (solid, liquid, gas) at varying temperatures and pressures. It helps to visualize the conditions under which a substance can exist in different states and the transitions between these states, such as melting, boiling, and sublimation.

Triple Point

The triple point of a substance is the specific temperature and pressure at which all three phases (solid, liquid, and gas) coexist in equilibrium. For CO₂, this occurs at a unique set of conditions, and understanding this concept is crucial for determining the phases present at atmospheric pressure.

Critical Point

The critical point is the end point of a phase equilibrium curve, beyond which the distinction between liquid and gas phases disappears. For CO₂, the critical point indicates the highest temperature and pressure at which it can exist as a liquid. Above this point, CO₂ can only exist as a supercritical fluid, which has properties of both gases and liquids.

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