Melting and boiling points: purity and precision
Chemical analysis • Purity, formulations, and chromatography
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Definition of melting point and boiling point
The melting point is the temperature at which a solid and its liquid form coexist in equilibrium under a specified pressure. The boiling point is the temperature at which the vapour pressure of a liquid equals the surrounding pressure, causing vigorous vapour formation. Pressure dependence limits both definitions because boiling-point values refer to a stated pressure (commonly 1 atm). Laboratory measurements require statement of the pressure or use of standard conditions.
How impurities affect melting and boiling behaviour
Presence of impurities lowers and broadens the melting range because foreign particles disrupt the solid crystal lattice, reducing the temperature at which melting begins and spreading the transition over several degrees. The cause is disruption of intermolecular order, and the effect is both a lower onset temperature and a wider melting interval. Impurities usually raise the boiling point (boiling-point elevation) because dissolved or mixed substances reduce the tendency of solvent molecules to escape into the vapour phase. The cause is a change in vapour pressure; the effect is a boiling point higher than the pure liquid under the same pressure.
Using melting/boiling data to distinguish pure substances from mixtures
A sharp melting point (narrow range, often 1 °C or less with good apparatus) indicates high purity. A depressed and broadened melting range indicates the presence of impurity. The cause-effect relationship is that impurities disrupt lattice order, so a mixed sample melts over a wider temperature interval and at a lower onset. A boiling point measured close to a known pure value and stable during repeated heating indicates purity. Significant deviation above the known boiling point or inconsistent readings suggests contamination, incorrect pressure, or experimental error. Comparison with reliable reference data at the same pressure provides the diagnostic check.
Practical limitations when using melting/boiling points
Heating rate, sample mass, capillary packing (for melting), calibration of thermometer, and atmospheric pressure cause measurement variation. Faster heating broadens the observed melting range and can give an artificially high boiling point because of thermal lag. The cause is non-equilibrium temperature gradients; the effect is reduced measurement accuracy. Small amounts of volatile impurity can evaporate during a boiling test, altering the composition and misleading interpretation. Instrument precision sets the smallest meaningful change that can be reported; reporting more digits than instrument precision misrepresents certainty.
Numerical precision and significant figures rules
Measured temperatures require reporting to the precision of the instrument. The smallest division on the thermometer or digital resolution defines the appropriate number of decimal places. An uncertainty estimate equals roughly ± half the smallest scale division. In calculations, multiplication and division require reporting to the same number of significant figures as the least precise quantity. Addition and subtraction require reporting to the least number of decimal places. The cause is propagation of measurement uncertainty; the effect is loss of precision in the final reported value. Rounding must follow standard rules and avoid implying greater accuracy than measurements allow.
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