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The triple point of a substance is a specific condition of temperature and pressure at which all three phases (solid, liquid, and vapor) coexist in thermodynamic equilibrium. Here’s a detailed explanation:


  1. Triple Point: The unique set of conditions (specific temperature and pressure) at which the three phases of a substance (solid, liquid, and vapor) are in equilibrium. At the triple point, a substance can simultaneously exist as a solid, liquid, and gas.


  1. Fundamental Thermodynamic Concept: The triple point is a fundamental concept in thermodynamics and physical chemistry, providing key insights into the behavior of substances under different conditions.
  2. Reference Point: It serves as a reference point for calibrating thermometers and other temperature-measuring devices. The triple point of water, for example, is used to define the Kelvin temperature scale.


  1. Equilibrium: At the triple point, the rates of phase transitions between solid, liquid, and vapor are equal, meaning there is no net change in the amount of each phase.
  2. Specific Conditions: Each substance has its own unique triple point. For instance:
  • Water: The triple point of water is at 0.01°C (273.16 K) and 611.657 pascals (0.00604 atm).
  • Carbon Dioxide: The triple point of CO₂ is at -56.6°C (216.55 K) and 518 kPa (5.11 atm).


  1. Metrology: The precise conditions of the triple point of water are used in the definition of the Kelvin temperature scale, making it a critical reference in temperature measurement.
  2. Phase Diagrams: The triple point is a crucial feature on a phase diagram, representing the intersection of the curves that separate the different phases (solid, liquid, and gas).
  3. Material Science: Understanding the triple point of materials helps in studying phase transitions and the properties of substances under varying conditions.

Visual Representation:

  • Phase Diagram: A phase diagram typically plots pressure against temperature and shows the regions where solid, liquid, and gas phases exist. The triple point is represented by a specific point where the three phase boundaries intersect.

The triple point is a unique and critical concept in thermodynamics, representing the specific conditions of temperature and pressure where a substance exists in equilibrium as a solid, liquid, and vapor. It is used as a reference point in scientific measurements and provides important insights into the phase behavior of substances.

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