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Melting and boiling points: purity and precision

Chemical analysis • Purity, formulations, and chromatography

Key concepts

What you'll likely be quizzed about

  • The melting point is the temperature at which a solid and its liquid form coexist in equilibrium under specified pressure.
  • The boiling point is the temperature at which the vapor pressure of a liquid equals the surrounding pressure, resulting in vigorous vapor formation.
  • Both definitions depend on pressure; boiling-point values refer to a stated pressure (commonly 1 atm), and laboratory measurements should note the pressure or use standard conditions.

Flashcards

Test your knowledge with interactive flashcards

When to use significant figures vs decimal places

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Use significant figures for multiplicative calculations and decimal places for additive calculations to match uncertainty propagation rules.

Key notes

Important points to keep in mind

Melting-point depression and range broadening indicate impurity due to lattice disruption lowering and spreading the melting transition.

Boiling-point elevation occurs when impurities reduce vapor pressure, resulting in higher boiling points than the pure substance at the same pressure.

Report temperatures to instrument precision; include an uncertainty estimate (approximately ± half the smallest division).

For multiplication/division, final significant figures match the least significant figures among inputs; for addition/subtraction, final decimal places match the least among inputs.

Rapid heating and larger samples lead to thermal lag, producing broader and less accurate melting/boiling transitions.

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