At triple point the substance co-exists in 1. Liquid phase 2. Solid phase 3. Vapour phase Select the correct answer using the code given below:

Correct — D, 1, 2 and 3. The triple point of a substance is the one combination of temperature and pressure at which its solid, liquid and vapour phases sit in equilibrium together. On a phase diagram it is the single point where the fusion, vaporisation and sublimation curves meet, and all three phases are present at once, so every one of the three listed phases belongs in the answer.

• (a)1 only — Naming the liquid alone describes an ordinary point inside the liquid region of the phase diagram, not the triple point.
• (b)1 and 2 only — Solid together with liquid describes any point along the fusion curve — ordinary melting. The triple point is where that curve meets two others.
• (c)2 and 3 only — Solid together with vapour describes the sublimation curve, where dry ice sits at atmospheric pressure. Again it is a line on the diagram, not the point where three lines cross.

A phase diagram plots pressure against temperature and divides the plane into solid, liquid and vapour regions. The boundaries between regions are lines along which two phases coexist. The triple point is the unique intersection at which all three boundary lines meet, and it is fixed for a given substance — a property, not a setting.

Because it is fixed and reproducible, the triple point of water was for many years the anchor of the temperature scale: the kelvin was defined so that this point sits at exactly 273.16 K, at a pressure of about 611.657 pascals, which is far below atmospheric. That is why water does not show its triple point in an open beaker. Since the redefinition of the SI base units in 2019 the kelvin is fixed instead through the Boltzmann constant, but the numerical value at the triple point of water is unchanged for every practical purpose. Do not confuse the triple point with the critical point, beyond which liquid and vapour cease to be distinguishable at all.

• At the triple point the solid, liquid and vapour phases of a substance coexist in equilibrium.
• It is the single intersection of the fusion, vaporisation and sublimation curves on a pressure-temperature diagram.
• For water it lies at 273.16 K and about 611.657 pascals, well below atmospheric pressure.
• The value 273.16 K defined the kelvin until the 2019 redefinition of the SI units through the Boltzmann constant.
• The critical point is a different landmark entirely — above it the liquid and vapour phases become indistinguishable.

• Confusing the triple point with the critical point.
• Assuming a triple point can be seen in an open vessel; for water the pressure required is far below atmospheric.
• Reading the question as asking which phases can coexist in general rather than which coexist at this particular point.

A definition item in statement-code form. Once the phrase 'triple' is connected to three phases, the code is settled.

No directly related past PYQ was found.

• practice — not a real PYQ

  The triple point of water corresponds to a temperature of

  • (a)0 K
  • (b)273.16 K
  • (c)373.15 K
  • (d)100 K

  Answer(b) 273.16 K — at a pressure of about 611.657 pascals, where ice, water and water vapour coexist.

• practice — not a real PYQ

  Solid carbon dioxide passes directly into vapour at atmospheric pressure. This process is called

  • (a)condensation
  • (b)sublimation
  • (c)fusion
  • (d)vaporisation

  Answer(b) sublimation — the solid-vapour boundary of carbon dioxide lies above atmospheric pressure, so no liquid stage appears.