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Triple Point Of Water

Triple Point Of Water

Vishal kumarUpdated on 02 Jul 2025, 07:35 PM IST

Imagine a place where it can be in the solid, liquid, and gaseous forms all at once. This kind of special condition is what is known as the triple point of water. It is a temperature-pressure characteristic that ice, liquid water, and water vapour coexist in perfect harmony with one another. The triple point of water helps scientists study the properties of substances and is utilized in the science of thermodynamics. Knowing this concept enables us to understand more about things regarding the behaviour of matter under different conditions.

Triple Point Of Water
Triple Point Of Water

Some important terminologies

  • Melting (or fusion) /freezing (solidification): The phase change of solid to liquid is called melting or fusion and the reverse phenomenon is called freezing or solidification.
  • Vaporisation / liquefication (condensation) : The phase change from liquid to vapour is called vaporisation. The reverse transition is called liquefication or condensation.
  • Sublimation: Sublimation is the conversion of a solid directly into vapours. So, in this, the solid is directly converted to vapour without entering into the liquid phase. The best example of this is the burning of Camphor.

Triple Point

If we plot a graph between pressure and temperature for any material. Then there are three curves form on this graph, they are - fusion curve, vaporisation curve and sublimation curve. The following graph shows the variation of pressure with temperature of the water -

Now, some description of the curves -

(i) Sublimation curve which connects points at which vapour (V) and solid (S) exist in equilibrium.
(ii) Vapourization curve which shows vapour and liquid (L) existing in equilibrium.
(iii) Fusion curve which shows liquid and solid existing in equilibrium.

The three curves meet at a single point which is called the triple point.

A triple point is that point for a substance where all three phases co-exist in equilibrium.

For water - A triple point exists at - Pressure = 0.0062 bar or, 62 Pascal

Temperature = 0.01oC or, 273.16 K

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Summary

The triple point of water is the temperature and pressure at which, simultaneously, ice, liquid, and water vapour can coexist in stable equilibrium. This happens exactly at a temperature of 0.01 degrees Celsius, under 611.657 pascals of pressure. The triple point has been quite an important basic concept in science because it provides a very accurate reference point for temperature and pressure measurements.

Frequently Asked Questions (FAQs)

Q: Can you describe how the behavior of water molecules differs in each phase at the triple point?
A:
At the triple point, water molecules in the solid phase vibrate in fixed positions, in the liquid phase they flow and interact with neighboring molecules, and in the gas phase they move freely with minimal interaction. The coexistence of these behaviors at a single point makes the triple point unique.
Q: How does the triple point of water relate to the concept of thermodynamic stability?
A:
The triple point represents a unique state of thermodynamic equilibrium where all three phases are equally stable. Any deviation from this point will result in one or more phases becoming more thermodynamically favorable.
Q: Can you explain how the concept of enthalpy relates to the processes occurring at the triple point?
A:
At the triple point, the enthalpies of all three phases are interrelated. Any phase transition involves an enthalpy change, but the overall system enthalpy remains constant as long as the three phases coexist in equilibrium.
Q: What role does quantum mechanics play in understanding the behavior of water at its triple point?
A:
Quantum mechanics plays a role in understanding the molecular interactions and energy states of water molecules at the triple point. It helps explain the precise conditions required for the coexistence of the three phases.
Q: Can you explain the concept of the triple line in relation to the triple point?
A:
The triple line is a line on a 3D phase diagram where three phases coexist. The triple point is a single point on this line for a pure substance. For mixtures, the triple line represents a range of compositions where three phases can coexist.
Q: How does the concept of phase coexistence at the triple point relate to Le Chatelier's principle?
A:
Le Chatelier's principle applies at the triple point: any small change in conditions (temperature or pressure) will cause the system to shift to counteract that change, maintaining the coexistence of the three phases as long as possible.
Q: How does the triple point of water relate to the concept of critical opalescence?
A:
Critical opalescence occurs near the critical point, not the triple point. However, both points represent unique states where phase boundaries become less distinct. At the triple point, this manifests as the coexistence of three distinct phases.
Q: What role does the triple point play in understanding supercritical fluids?
A:
While the triple point and supercritical fluids are distinct concepts, understanding the triple point helps in grasping phase behavior. Supercritical fluids exist above the critical point, where the distinction between liquid and gas phases disappears.
Q: How does the concept of degrees of freedom in thermodynamics apply to the triple point?
A:
At the triple point, a pure substance has zero degrees of freedom according to the Gibbs phase rule. This means that both temperature and pressure are fixed for the three phases to coexist, leaving no independent variables.
Q: How does the concept of chemical equilibrium apply at the triple point?
A:
At the triple point, all three phases are in chemical equilibrium with each other. This means that the rates of transition between any two phases are equal in both directions, resulting in no net change in the amount of each phase.
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