The melting point of a substance is the temperature range over which a crystalline solid converts into a liquid state. This physical property is a fundamental characteristic, typically measured under atmospheric pressure. Determining this range is important in chemistry because it helps assess the identity of an unknown compound and the purity of a known substance. Since every pure compound has a unique and sharp melting point, this measurement serves as a reliable identification tool.
Preparing the Sample for Testing
Inaccurate results often stem from poor sample preparation, making this step important for reliable measurement. The solid material must be completely dry; residual solvents or moisture act as impurities, causing the observed melting range to be lower and broader than the true value. Before loading, the sample should be ground into a fine powder using a mortar and pestle. This fine consistency ensures uniform heat transfer and allows for consistent, dense packing.
To load the sample, powder is collected into a thin-walled, glass capillary tube sealed at one end. The powder must then be compacted down to the sealed end to create a dense column, typically 2 to 4 millimeters high. This compaction is often achieved by repeatedly dropping the capillary tube, sealed-end first, through a long glass tube onto a hard surface. A sample height exceeding 4 millimeters can result in an artificially broad melting range due to inconsistent heating.
Standard Equipment and Techniques
The standard method employs a device designed to heat the sample within a capillary tube at a controlled rate. One traditional apparatus utilizes a Thiele tube, a glass setup containing a high-boiling oil bath and a thermometer. The tube’s unique shape promotes convection currents in the oil when heated, ensuring uniform temperature distribution around the sample and the thermometer.
More modern instruments, known as capillary melting point apparatus, use an electrically heated metal block instead of an oil bath. These devices offer greater convenience and precision, often featuring a viewing eyepiece or digital camera to observe the melt. Digital models incorporate internal sensors to automatically detect the melting point and temperature controllers to apply a highly stable heating rate. The apparatus holds the prepared capillary tube close to a temperature sensor to ensure the recorded temperature accurately reflects the sample’s environment.
Executing the Measurement Procedure
Once the capillary tube is prepared, it is placed into the designated slot within the melting point apparatus, adjacent to the temperature sensor. If the approximate melting point is unknown, an initial rapid scan is performed using a fast heating rate (e.g., 10 °C per minute) to quickly determine a rough range. After this preliminary melt, the apparatus must be cooled significantly, and a fresh sample is used for the accurate determination.
For the final, precise measurement, the instrument’s starting temperature is set approximately 10 to 15 °C below the expected melting point. The heating rate is then slowed dramatically to a precise rate, typically 1 to 2 °C per minute. This slow rate is important for ensuring the thermometer and sample are in thermal equilibrium, preventing the temperature increase from outpacing heat transfer into the tiny sample.
The measurement requires precise observation of two temperatures to define the melting range. The first temperature, the “start” of the melt, is recorded when the first distinct drop of liquid appears in the compacted solid. The second temperature, the “end” of the melt, is recorded when the last trace of solid material has completely converted to a clear liquid. The resulting range, expressed as two temperatures (e.g., 149.5–150.5 °C), is then used for analysis.
Analyzing the Observed Melting Range
The melting range obtained is a strong indicator of a substance’s purity. A pure, crystalline compound typically melts over a narrow range, often 1 to 2 °C or less. This sharpness occurs because the ordered crystal lattice requires a specific amount of energy to fully transition to a liquid.
Impurities disrupt this order, causing the observed melting range to be both lower and broader than the true value, a phenomenon known as melting point depression. Contaminants weaken the intermolecular forces within the solid structure, causing the range to widen and the initial melting temperature to drop significantly.
To definitively confirm the identity of an unknown substance, the mixed melting point technique is used. The unknown sample is mixed thoroughly with a known, pure sample of the suspected compound. If the mixed sample exhibits a narrow and undepressed melting range, the two substances are identical. Conversely, if the mixture shows a broadened and lowered melting range, the compounds are different, and the initial identification was incorrect.