【Daily Question No. 76】March 16, 2020

Correct. Boiling range is a temperature range, not some phase transition process. Strictly speaking, the melting and boiling points of a substance are not a single point, but rather a temperature range known as the melting and boiling range. Taking the melting range as an example, the two limits of this range are referred to as the initial melting temperature and the final melting temperature; the initial melting temperature is the temperature at which a substance begins to melt, while the final melting temperature is the temperature at which the substance melts completely. For crystals, the purer the substance, the shorter its melting and boiling ranges. Pure liquid compounds have a very narrow boiling range during distillation (0.5–1°C), whereas mixtures do not have a fixed boiling point; in other words, their boiling ranges are wider. For example, the boiling range of gasoline is 40–150°C. For crystal samples, the melting range and boiling range can qualitatively serve as criteria for the purity of a substance. 　　The thermometer readings at the start of distillation and at the moment the last drop is collected represent the boiling range of that fraction