Final answer:
To determine if a liquid is a pure substance or a mixture, observe its boiling or melting point and use chromatography, as a pure substance has a fixed point and produces one result, respectively.
Step-by-step explanation:
One way to determine if a liquid is a pure substance or a mixture is by examining its boiling or melting point. A pure substance has a sharply defined melting or boiling point. Conversely, a mixture will have a range of temperatures over which it boils or melts due to the presence of multiple substances with different boiling and melting points. Another method involves the use of chromatography, which separates a mixture into its individual components. A pure substance will produce a single component in chromatography, while a mixture will result in several distinct substances being separated.