Triple Point of a Substance

Every substance has triple point. Triple point of a substance is a point at which it exists in solid, liquid and vapor phase.

Triple point is the point of intersection of sublimation line, vaporization line and fusion line on Pressure vs Temperature (P-T) phase diagram where all three phases line and fusion line on P-T diagram where all three phases coexist in equilibrium.

On other property diagrams (P-V and T-V), such a conditions are represented by a line called triple line.

Triple point is a point  in P-T diagram. It is a line in P-v diagram and it is a triangle in U-v diagram.

Fusion line have negative(-ve) slope. It is due to to anomalous behavior of water i.e, the melting point of increasing pressure.

Pressure vs temperature phase diagram of Triple point

Pressure vs temperature phase diagram of Triple point.
Image source: Wikimedia Commons

Only three substances (water, Antimony and Bismuth) have negative (-ve) slope for fusion lines whereas all remaining substances have positive (+ve) slope.

Changes of states:

  1. If any substance heated below its triple point pressure, solid becomes vapor and process is called sublimation.
  2. If any substance is heated above its triple point pressure, solid turns to liquid state and liquid turns to vapor state.
  3. Isothermal compression less than triple point temperature makes substance go from vapor to liquid state.
  4. Isothermal compression greater than triple point temperature makes substance go from vapor to liquid state.
  5. Isothermal expansion less than triple point temperature makes substance go from solid to vapor state.
  6. Isothermal expansion greater than triple point temperature makes substance go from liquid to vapor substance.

The value of specific entropy of a substance at triple point is zero.

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