Question
Why is it necessary to heat slowly when the temperature is near the melting point?
Answer
Near the melting point, the sample must be heated slowly so that the sample, capillary, heating block or bath, and temperature sensor have enough time to approach thermal equilibrium.
If the apparatus is heated too rapidly, the displayed temperature can rise faster than the sample itself warms. This creates thermal lag between the heating block or sensor and the solid inside the capillary.
That lag distorts the measurement. MIT OpenCourseWare notes that when the temperature is increased too rapidly, the capillary does not have enough time to equilibrate with the heated block; the measured melting temperature can appear too high, and the observed melting range can appear narrower than it actually is.
Slow heating also gives the observer time to identify the two endpoints correctly:
- the temperature at which the first droplet of liquid appears;
- the temperature at which the last crystal disappears.
A common capillary procedure is therefore to heat rapidly while well below the expected transition, then reduce the rate to about 1–2 °C per minute near the melting range, subject to the instrument's own instructions.
Evidence boundary
This explanation applies to standard capillary melting-point measurements. Exact ramp-rate settings depend on the apparatus and laboratory procedure; the university and laboratory references checked here commonly use about 1–2 °C/min near the expected melting point. No compound-specific melting point is inferred.
Sources
These references support the concepts and methods used in the explanation above.