Physics · Thermal Properties Of Matter · NEET
No. They are close but not equal. The normal melting point of ice is 0 degree C (273.15 K) and it happens at 1 atmosphere pressure (about 1.01 x 10^5 Pa). The triple point is at 0.01 degree C (273.16 K) and at a very low pressure of only 6.11 x 10^-3 Pa. At the melting point only solid and liquid coexist, but at the triple point all three phases - solid, liquid and vapour - coexist together.
273.16 K is an exact, defined value. Scientists chose the triple point of water as one fixed reference to build the Kelvin scale, and they assigned it the exact number 273.16 K by choice. So this value has no measurement error. The number 273.15 K (0 degree C) is the melting point at 1 atm and is a measured, slightly different value.
A phase diagram is a graph with pressure P on one axis and temperature T on the other. It splits the plane into a solid region, a liquid region and a vapour region. Three curves separate these regions: the fusion curve (AO, solid and liquid coexist), the vaporisation curve (CO, liquid and vapour coexist) and the sublimation curve (BO, solid and vapour coexist). The one point where all three curves meet is the triple point.
Exactly three: solid ice, liquid water and gaseous water vapour, all in equilibrium at the same time. This is the only single set of temperature and pressure values where this three-way balance is possible for water.
The triple point is where solid, liquid and vapour coexist (273.16 K, 6.11 x 10^-3 Pa for water). The critical point is the end of the vaporisation curve; above it the liquid and vapour become one single phase and you cannot tell them apart. They are two different special points on the phase diagram and should not be confused.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
273.16 K, which equals 0.01 degree C.
6.11 x 10^-3 Pa, a very low pressure compared to normal atmospheric pressure of about 1.01 x 10^5 Pa.
It is a fixed, reproducible reference that is used to define the Kelvin temperature scale, because it always occurs at exactly the same temperature and pressure for a pure substance.
The sublimation curve (BO). Along it, solid and vapour phases coexist without any liquid.
Yes, every pure substance has its own unique triple point where its solid, liquid and vapour phases coexist, though the temperature and pressure values differ for each substance.
