Triple point

In thermodynamics, a triple point is a very specific state of matter. It occurs at a unique temperature and pressure for any given substance. At this exact point, three different phases of that substance coexist in thermodynamic equilibrium. This means the solid, liquid, and gas phases all exist together in a stable balance.

Scientists use these points to understand how matter transitions between different forms. Each substance has its own unique triple point that serves as a fundamental physical constant.

To understand the mechanism, we must look at phase curves. These curves represent the boundaries where a substance changes state, such as melting or boiling. For most substances, the sublimation, fusion, and vaporization curves meet at a single intersection. This intersection is the triple point.

At this intersection, the substance is simultaneously undergoing phase changes. If you change the pressure or temperature even slightly, the equilibrium is broken. The substance will then settle into one or two of its phases instead of three.

Most substances follow a standard pattern, but some are more complex. A triple point can involve more than one solid phase if the substance has multiple polymorphs. Polymorphs are different structural forms of the same solid material. Helium-4 is an unusual example because it lacks a sublimation curve. This means it has no triple point where its solid phase meets its gas phase. Instead, helium-4 has several unique points involving a state called a superfluid. These include a vapor-liquid-superfluid point and various solid-liquid-superfluid combinations.

The concept was formally introduced in the late 19th century. James Thomson, the brother of Lord Kelvin, first mentioned the term "triple point" on August 3, 1871. He presented his findings at a British Association Meeting in Edinburgh. Thomson showed how this specific point of pressure and temperature belongs to three important curves. He identified it as the mathematical intersection of those curves. His work provided a way to categorize how different states of matter relate to one another.

Triple points are essential for high-precision science and measurement. They are used to define the International Temperature Scale of 1990, or ITS-90. This scale uses the triple points of several substances to set temperature standards. These include hydrogen, neon, oxygen, argon, mercury, and water. For example, the triple point of hydrogen is 13.8033 K. The triple point of mercury occurs at a temperature of -38.83 °C and a pressure of 0.165 mPa.

Water provides one of the most important examples in science. The triple point of water occurs at approximately 273.16 K, 0.01 °C, and a vapor pressure of 611.657 Pa. Before 2019, this point was used to define the kelvin, the base unit of temperature. While the definition of the kelvin changed in 2019, the triple point remains a vital empirical constant. Water also shows complex behavior at high pressures. There are 15 known phases of ice, which create many different triple points. One such point exists at 251 K and 210 MPa, where ice Ih, ice III, and liquid water coexist.

To achieve such precision, scientists use triple-point cells to calibrate thermometers. These cells are filled with highly pure chemical substances. The purity is often referred to as "six nines," meaning the substance is 99.9999% pure. For water, scientists use a specific isotopic composition known as VSMOW. This is because small variations in isotopes can change the triple point value. By using these stable, reproducible points, scientists can ensure that temperature measurements are accurate all over the world.