Tin (Sn) is a soft, silvery-white metal known for its malleability and resistance to corrosion. The temperature at which liquid tin becomes solid, or the freezing point, is the central characteristic driving many of its modern applications. This temperature is a specific, immutable value that has been precisely measured and adopted as a standard in global science and industry.
The Precise Value and Definition of Freezing
The freezing point of pure tin is a constant value representing the temperature at which the substance shifts from its liquid phase to its solid phase. For a high-purity element, this transition temperature is identical to the melting point. The internationally accepted value for this phase change is 231.928°C (degrees Celsius), defined with high precision for scientific use.
This temperature translates to 449.47°F (degrees Fahrenheit) or 505.08 K (Kelvin). The precision of this value relies heavily on the purity of the metal sample, as even minor contaminants can lower the freezing temperature. Achieving this exact, highly reproducible temperature requires tin that is 99.9999% pure, which is the standard used in metrology.
During the transition, the temperature remains constant until all the material has fully changed state. This period of thermal stability is known as a freezing plateau. The existence of this plateau is a defining characteristic of a pure substance’s phase transition, providing a fixed, reliable temperature reference.
Tin as a Thermometric Calibration Standard
The highly stable and reproducible freezing point of tin gives it a specialized role in global temperature measurement. This specific temperature is designated as a defining fixed point on the International Temperature Scale of 1990 (ITS-90). Fixed points are intrinsic standards, providing a foundation for calibrating the world’s most accurate thermometers.
Metrology laboratories realize this fixed point using a specialized apparatus called a fixed-point cell. These cells contain a large sample of ultra-pure tin, often sealed within a graphite crucible and a protective sheath. The cell is placed in a furnace, melted, and then slowly cooled to generate the freezing plateau.
When the tin begins to solidify, the Standard Platinum Resistance Thermometer (SPRT) being calibrated is inserted into the cell. The thermometer’s resistance is measured against the stable temperature of 231.928°C realized by the phase change. The tin fixed-point cell is an absolute standard, ensuring high-accuracy instruments are compliant with the ITS-90 scale.
Key Industrial Applications of Low Melting Point Tin
Tin’s relatively low freezing point makes it highly valuable for many large-scale manufacturing processes. This low transition temperature means the metal can be easily liquefied and solidified for practical use. The most common application is in soldering, where tin is a primary component in alloys used to join electronic and metal parts.
The low melting temperature of these tin-based solders allows connections to be made without exposing sensitive components to excessive heat. The ability to transition from liquid to solid below 250°C is fundamental to modern electronics manufacturing, ensuring a strong, reliable bond is formed quickly and efficiently.
Tin is also a component in various fusible alloys, which are designed to melt at specific, low temperatures. These alloys are used in safety applications, such as the thermal links in fire sprinkler systems and safety fuses. The precise composition of the alloy determines the exact temperature at which the link will melt and activate the safety mechanism.
A specialized application is found in the production of high-quality flat glass, known as the float glass process. Molten glass is poured onto a bath of molten tin, which remains stable and liquid at the required processing temperature. The low melting point of tin, combined with its high boiling point, allows it to form a perfectly level surface for the glass to solidify, creating a uniform, flawless sheet.