In the chemistry lab, some of the most profound insights come from the simplest experiments. Melting point determination stands as a cornerstone technique—seemingly straightforward yet rich with thermodynamic meaning. This guide breaks down exactly how it works and why it remains indispensable.

I. Core Definitions

Melting Point : The temperature at which a solid coexists in equilibrium with its liquid under standard pressure. It is a defining physical property.
Melting Range : The temperature interval from when a substance starts to melt (initial melt) to when it completely liquefies (final melt).
The Key Insight : A pure crystalline compound has a sharp, reproducible melting point with a very narrow range (typically 0.5–1.5°C). The presence of impurities lowers the observed melting point and broadens the melting range significantly.

II. The Underlying Principle: Why It Reveals Purity & Identity

1. The Thermodynamic Basis

At the melting point, the vapor pressures of the solid and liquid phases are equal, allowing coexistence.
When an impurity is introduced, it disrupts the perfect crystal lattice. According to Raoult's Law and freezing point depression, the impurity reduces the solid's tendency to form, lowering the melting point.
Furthermore, the impurity causes the solid to melt gradually over a wider temperature range, resulting in a broad melting range.

2. The Experimental Principle

The essence of the method is: "Observe the physical change of the sample under controlled heating and record the corresponding temperature."
Heating Control : Heating must be slow and uniform (1-2°C/min near the melt). Why?

• Heat transfer into the capillary takes time.
  

• A thermal lag exists between the thermometer and the sample. Heating too fast gives falsely high readings.

3. Visual Observation 

• Initial Melt : Crystals round at edges; first liquid appears. → Onset Temperature

• Final Melt : All solid disappears into clear liquid. → Final Temperature

• The interval is the Melting Range.
  

III. Melting Behavior: A Comparison

IV. Key Applications

1. Purity Assessment : A broad melting range immediately signals impurities.
   

2. Compound Identification  - The Mixed Melting Point Method:
   

   • Mix unknown with a suspected known pure compound.

   • No depression/broadening? → Likely the same compound.

   • Significant depression/broadening? → Different compounds.

   • Scientific Basis: Different compounds act as impurities to each other.

Summary

The principle of melting point determination lies at the intersection of thermodynamics, precise measurement, and practical observation. It connects microscopic phase behavior to a macroscopic temperature reading, elegantly combining theory (freezing point depression), technique (slow heating), and application (purity check, identification).
In short: By watching when and how a substance melts, we learn what it is and how pure it is.