Q.Answer the following:

(a) The triple point (ice + water + vapour coexisting, $273.16$ K) is a single, reproducible state needing no pressure reference. The old fixed points - melting ice and boiling water - both depend on atmospheric pressure (boiling point especially, e.g. water boils near $70^\circ$C on Everest), making them unreliable universal standards.

(b) The Kelvin scale is defined using exactly two reference points: the triple point of water, fixed by definition at $273.16$ K, and the zero of the scale itself. Since the triple point already supplies one of those two defining points, the other must be the scale's own origin — absolute zero ($0$ K), the theoretical lower limit of temperature at which a substance's thermal molecular motion is minimum.

(c) The historical ice point ($0^\circ$C at 1 atm) measures $273.15$ K, which is $0.01$ K below the triple point ($273.16$ K, defined at a much lower, fixed pressure). Celsius kept its zero pinned to the (pressure-dependent) ice point rather than redefining it, so the Kelvin-to-Celsius conversion uses $273.15$, not $273.16$. …

(a) Why the triple point, and what was wrong with the old fixed points

The triple point of water is the unique temperature and pressure ($273.16$ K, $611.657$ Pa) at which ice, liquid water, and water vapour all coexist together in equilibrium. It is a single point on water's phase diagram, not a line - so it needs no external pressure reference; the system fixes both its own temperature and pressure simultaneously.

The old Celsius fixed points - the melting point of ice ($0^\circ$C) and the boiling point of water ($100^\circ$C) - both depend on the surrounding atmospheric pressure. Water's boiling point in particular shifts noticeably with altitude (it boils near $70^\circ$C atop Mount Everest); the melting point shifts too, though by much less. Both therefore need a pressure to be specified alongside the temperature, and both vary if that pressure isn't controlled - making them less reliable as universal standards than the self-defining triple point.

(b) The other fixed point on the Kelvin scale

The Kelvin scale is an absolute scale: its zero, $0$ K, is absolute zero - the lower limit of temperature, where classical thermal motion ceases. The upper reference is the triple point of water, fixed at $273.16$ K. So the Kelvin scale needs only these two anchors: absolute zero at the bottom, and the triple point as the single experimentally-realizable calibration point.

(c) Why $273.15$, not $273.16$ …

The unifying idea across all four parts is that a good temperature standard must depend on nothing except temperature. (a) Melting/boiling points are single-phase transitions at a chosen pressure, silently assuming 'standard atmosphere' as a hidden second variable — change the pressure (as on a mountain) and the 'fixed' point moves; the triple point is special because three coexisting phases pin down pressure automatically, with no external choice needed. (b)–(c) Absolute zero anchors the other end of the scale, and the tiny $0.01$ K gap between $273.15$ and $273.16$ is simply the historical ice point sitting fractionally below the later-adopt …