Understanding the Kelvin Scale Reference Point

The Kelvin scale is the base unit of thermodynamic temperature in the International System of Units (SI). It is named after physicist William Thomson, 1st Baron Kelvin.

The Kelvin Scale and its Definition

Historically, temperature scales were defined based on easily reproducible points like the freezing and boiling points of water. However, these points depend on pressure, making them less ideal for defining a fundamental thermodynamic scale.

To establish a more precise and fundamental definition for the thermodynamic temperature scale, the Tenth General Conference on Weights and Measures in 1954 made a significant decision regarding the definition of the Kelvin scale. They chose a single, fixed point as the reference:

• This chosen reference point is the triple point of water.

What is the Triple Point of Water?

The triple point of water is the unique temperature and pressure at which the three phases of water — solid ice, liquid water, and water vapor — coexist in thermal equilibrium. This point occurs at a specific temperature of $0.01 \text{ °C}$ and a specific pressure of $611.657 \text{ pascals}$ ($0.006037 \text{ atmospheres}$).

The temperature of the triple point of water was defined by the 1954 conference as exactly $273.16 \text{ K}$ (which is $0.01 \text{ °C}$). This fixed point, along with absolute zero ($0 \text{ K}$), defined the scale. Absolute zero is the theoretical temperature at which the atoms and molecules of a substance have the lowest possible energy.

Comparing with Other Options

Let's consider why the other options were not chosen as the fundamental reference point in 1954:

• Freezing point of water: This is the temperature at which water freezes into ice. It is $0 \text{ °C}$ ($273.15 \text{ K}$) at standard atmospheric pressure. However, it varies slightly with pressure.
• Boiling point of water: This is the temperature at which water boils into vapor. It is $100 \text{ °C}$ ($373.15 \text{ K}$) at standard atmospheric pressure. Like the freezing point, it is highly dependent on pressure.
• Critical point of water: This is the temperature and pressure above which distinct liquid and gas phases of water do not exist. It's a much higher temperature ($374 \text{ °C}$) and pressure. While unique, it's not suitable as a defining point for the common temperature range or for easy realization in standard laboratory settings compared to the triple point.

The triple point of water is preferred because it is a single, stable, and highly reproducible state that is relatively easy to achieve experimentally, making it an excellent fundamental reference point for a temperature scale.

Therefore, as per the Tenth General Conference on Weights and Measures in 1954, the reference point temperature chosen for the Kelvin scale is the triple point of water, defined as $273.16 \text{ K}$.