How does the normal boiling point serve as a characteristic property of a pure liquid substance?

The normal boiling point is a critical characteristic property of a pure liquid because it provides a specific temperature at which the liquid's vapor pressure equals the atmospheric pressure at 1 atm. At this point, a pure substance transforms from its liquid state to its gaseous state, and this temperature is unique to each pure substance under given pressure conditions.

This distinctive boiling point is vital because it allows for the identification of substances. When a substance is being tested, if its boiling point matches the known normal boiling point of a pure substance, it can confirm the identity of the substance being analyzed. This is especially useful in laboratories, where purity can often be assessed through boiling point measurements.

In contrast, the boiling point of a substance can vary depending on external conditions, such as atmospheric pressure, which does not support its role as a characteristic property under standard conditions. Additionally, impurities in a liquid can affect its boiling point, usually raising it due to the colligative properties, which further distinguishes pure substances from mixtures. Thus, the normal boiling point specifically indicates the identity of a pure substance.