Sodium is a soft, silvery element classified as an alkali metal, known for its surprisingly low melting point. This element, represented by the symbol Na, is a soft solid at standard room temperature. The temperature at which sodium transitions from a solid to a liquid, or freezes, is approximately 97.72 degrees Celsius, which is significantly lower than most common metals. This allows sodium to be maintained as a liquid at temperatures just above the boiling point of water, a property that makes it valuable for specialized industrial uses. The low freezing point is directly related to the specific way sodium atoms bond together, which differs from the robust structures found in metals like iron or copper.
The Specifics of Sodium’s Freezing Point
The freezing point of pure elemental sodium is 97.72 degrees Celsius (207.9 degrees Fahrenheit) under normal atmospheric pressure. For any pure substance, the freezing point and the melting point are identical, representing the single temperature at which the solid and liquid phases exist in equilibrium. To achieve this precise measurement, the sodium sample must be highly purified, as even trace amounts of contaminants can alter the observed temperature by a process known as freezing point depression. When expressed on the absolute temperature scale, sodium’s freezing point corresponds to 370.87 Kelvin.
Understanding the Phase Transition
Freezing is a phase transition, where the substance changes from a less-ordered liquid state to a highly-ordered crystalline solid state. During this transition, the kinetic energy of the sodium atoms decreases sufficiently for the interatomic attractive forces to lock the atoms into fixed positions. As the liquid sodium crystallizes into its solid lattice structure, energy is released into the surroundings. This released energy is called the latent heat of fusion, and for sodium, its value is approximately 2.60 kilojoules per mole. The temperature of the material remains constant at 97.72 degrees Celsius until all of the liquid has solidified, because the energy being removed is entirely consumed by the change in phase, not by a drop in temperature.
In the liquid state, sodium atoms are constantly moving past one another, allowing the substance to flow and take the shape of its container. As the temperature drops to the freezing point, the atoms begin to arrange themselves into a repeating, three-dimensional lattice. The latent heat of fusion represents the energy required to break the bonds holding the solid lattice together during melting, or conversely, the energy released when those bonds form during freezing. This energy signature is specific to sodium and reflects the strength of the forces holding its solid structure in place.
Sodium’s Structure and Its Low Freezing Point
The reason sodium has a comparatively low freezing point lies in its atomic structure and the nature of its metallic bonding. Sodium (Na) is the eleventh element on the periodic table. As a Group 1 element, sodium is an alkali metal, characterized by having only one electron in its outermost energy shell. This single valence electron is loosely held and readily delocalized, meaning it contributes to a shared “sea of electrons” that holds the positive sodium ion cores together.
This metallic bond is considered weak because only one electron per atom is available to participate in the collective attraction between the atoms and the electron sea. Metals like iron or copper, in contrast, have two or more valence electrons participating in their metallic bonds, creating a much stronger electrostatic attraction. Therefore, less thermal energy is required to disrupt the relatively weak metallic bonds in sodium and transition it from a rigid solid to a flowing liquid. The large atomic radius of the sodium atom also contributes to the bond’s weakness, as the single valence electron is shielded by inner electron shells and is further from the positive nucleus.
Applications Utilizing Liquid Sodium
The low melting point and other physical characteristics of sodium make it a highly desirable substance for specialized technological applications where efficient heat transfer is paramount. Liquid sodium is widely used as a heat transfer fluid because it possesses an exceptionally high thermal conductivity, allowing it to quickly absorb and transport thermal energy. Furthermore, the metal has a very low viscosity, similar to that of water, which means it can be circulated through complex systems with minimal pumping power. These properties are particularly advantageous in high-temperature environments where conventional coolants, such as water or oil, would not be suitable.
One of the most significant industrial uses of liquid sodium is in advanced nuclear power generation, specifically in sodium-cooled fast reactors. In these reactors, liquid sodium is circulated through the core to remove intense heat. Because its boiling point is extremely high—around 882 degrees Celsius—the system can operate at high temperatures without the need for high-pressure containment. This ability to remain a liquid over a wide temperature range, from 97.72°C to 882°C, provides a massive operational margin and simplifies the engineering design of the reactor components.
Liquid sodium is also finding use in concentrated solar thermal power (CSP) systems, where it is employed to transfer heat from the solar receiver to a power block for electricity generation. The high thermal conductivity allows the solar plant to efficiently capture and store solar energy for extended periods, even after the sun has set. Its use in both nuclear and solar technologies demonstrates how this metal’s unique properties are leveraged to manage extreme heat loads and maximize energy conversion efficiency in modern power systems.