The melting point of an element is a fundamental physical constant defined as the temperature at which its solid and liquid phases exist in equilibrium, typically under standard atmospheric pressure. Determining this value is a routine technique in chemistry and materials science. It serves two primary functions: confirming the identity of an unknown element and assessing the purity of a known sample. A pure element exhibits a sharp, narrow melting point, while impurities cause the melting temperature to drop and the melting process to occur over a wider range.
Essential Equipment and Sample Preparation
The standard laboratory method uses a specialized melting point apparatus, which typically employs a heated metal block or an oil bath (like a Thiele tube) to control the temperature. These instruments include a precise thermometer or digital temperature probe and a viewport or magnification system to observe the sample. The sample is contained within a thin-walled glass capillary tube, sealed at one end.
For an accurate measurement, the solid sample must be prepared to ensure uniform heating. The element is first crushed into a very fine powder to promote efficient heat transfer. The open end of the capillary tube is dipped into the powder, inverted, and the sample is packed tightly into the sealed bottom. This is done by gently tapping the tube on a hard surface or dropping it through a longer, narrow glass tube.
The final packed height of the powder should be small, usually between two and three millimeters. Proper packing is important because a poorly packed or overly high column heats unevenly, leading to an artificially broad and inaccurate melting range. The prepared capillary tube is then placed into a dedicated slot within the apparatus, positioning the sample directly next to the temperature sensor for the most reliable reading.
Performing the Melting Point Determination
The apparatus must first be calibrated using known, highly pure reference substances to ensure the thermometer provides an accurate reading across the temperature range of interest. The entire process is performed twice: a preliminary run to find an approximate melting point, followed by a slower, precise run.
In the initial run, the apparatus is heated quickly until the sample melts, identifying a rough melting temperature. The second determination begins with the apparatus cooled down. The temperature is quickly raised to about 15°C below the expected melting point. At this point, the heating rate must be reduced to a slow, steady rate, ideally between 1°C and 2°C per minute.
This slow heating ensures the temperature reading accurately reflects the sample temperature. As the temperature rises, the sample is continuously observed through the viewing port. The first temperature recorded is the point at which the solid material begins to collapse or the first droplet of liquid is clearly visible.
The final temperature is recorded when the entire sample has turned into a transparent, homogenous liquid with no solid particles remaining. This pair of temperatures is reported as the melting range. For a pure element, this range should be extremely narrow, often less than a single degree Celsius. A broad range suggests the presence of impurities, which interfere with the crystal lattice structure.
Measuring Elements with Extremely High Melting Points
The standard capillary method is impractical for elements with extremely high melting points, such as tungsten (3422°C) or tantalum (3017°C). The glass or oil of the traditional apparatus cannot withstand the heat, and conventional thermocouples would melt or fail. These refractory elements require specialized high-temperature furnaces and non-contact measurement techniques.
One specialized technique involves heating a wire or small rod of the element via electrical resistance in a controlled environment, such as a vacuum or an inert gas-filled chamber. The current is gradually increased until the material melts, which is detected by a sudden break in the electrical circuit. The temperature of the glowing sample is measured remotely using optical pyrometry.
Optical pyrometry is a non-contact technique that determines temperature by measuring the intensity of the thermal radiation emitted by the hot object. The instrument compares the element’s brightness to a calibrated internal source or uses Planck’s law of black-body radiation to calculate the temperature. Specialized vacuum or induction furnaces are necessary to prevent the highly reactive elements from oxidizing or reacting with the surrounding atmosphere, which would contaminate the sample.