Triple Point

Definition

The triple point is a unique point on a substance's phase diagram where the solid, liquid, and gaseous phases of that substance coexist in equilibrium. At this specific temperature and pressure, all three phases can be present simultaneously.

5 Must Know Facts For Your Next Test

1. The triple point is a unique point on a substance's phase diagram where all three phases (solid, liquid, and gas) coexist in equilibrium.
2. At the triple point, the vapor pressure of the solid and liquid phases are equal, and the Gibbs free energy of all three phases is the same.
3. The temperature and pressure at the triple point are characteristic of a specific substance and are used as reference points for defining temperature and pressure scales.
4. The triple point of water occurs at 0.01°C (32.018°F) and 4.58 mmHg (0.6113 kPa), making it a crucial reference point for the Celsius temperature scale.
5. The existence of the triple point is a consequence of the Gibbs phase rule, which relates the number of degrees of freedom in a system to the number of phases and components present.

Review Questions

• Explain the significance of the triple point in the context of phase changes and latent heat.
  • The triple point is a unique point on a substance's phase diagram where the solid, liquid, and gaseous phases coexist in equilibrium. At this specific temperature and pressure, the substance can freely transition between all three phases without the need to overcome an energy barrier, such as latent heat. The triple point is an important reference point for understanding phase changes and the associated energy transformations, as it represents the conditions where the Gibbs free energy of the three phases is equal.
• Describe how the triple point is used to define temperature and pressure scales, and discuss the importance of this in scientific measurements.
  • The temperature and pressure at the triple point are characteristic of a specific substance and are used as reference points for defining temperature and pressure scales. For example, the triple point of water is used to define the Celsius temperature scale, where 0.01°C is set as the triple point temperature. This provides a precise and reproducible reference point that allows for the accurate measurement and comparison of temperatures across different scientific applications. Similarly, the pressure at the triple point is used to define pressure scales, ensuring consistent and reliable measurements across various fields of study.
• Analyze the relationship between the triple point, phase diagrams, and the Gibbs phase rule. Explain how these concepts are interconnected and their significance in understanding the behavior of substances.
  • The triple point is a direct consequence of the Gibbs phase rule, which states that the number of degrees of freedom in a system is equal to the number of components minus the number of phases plus two. At the triple point, the number of phases (solid, liquid, and gas) is equal to the number of components (the substance), resulting in zero degrees of freedom. This means that the temperature and pressure at the triple point are fixed and characteristic of the substance, as any change in these variables would result in the disappearance of one of the phases. The triple point is therefore a crucial reference point on a substance's phase diagram, as it represents the unique conditions where all three phases coexist in equilibrium, providing valuable insights into the phase behavior and transformations of the substance.

Related terms

Phase Diagram: A graphical representation that shows the equilibrium conditions (temperature and pressure) under which a substance can exist in different phases, such as solid, liquid, and gas.

Latent Heat: The energy released or absorbed by a substance during a phase change (e.g., solid to liquid, liquid to gas) at a constant temperature and pressure.

Phase Change: The transition of a substance from one state of matter (solid, liquid, or gas) to another, driven by changes in temperature and/or pressure.