Give three physical properties that identify a pure substance.

Pure substances can be recognized by several distinct physical properties that remain constant under specific conditions:

1. Fixed Melting Point: Every pure substance melts at a specific temperature. For example, pure ice always melts at 0°C (under normal pressure). Mixtures, however, melt over a range of temperatures because they contain multiple components.
2. Fixed Boiling Point: Similarly, pure substances have a definite boiling point. Pure water boils at 100°C at sea level. In contrast, salt water boils at a higher temperature because dissolved salt changes its composition.
3. Uniform Density: The density (mass per unit volume) of a pure substance is the same throughout a sample and remains constant under the same conditions. For instance, the density of pure gold is always 19.3 g/cm³.

Other measurable properties such as color, solubility, and electrical conductivity may also help identify a pure substance, but melting point, boiling point, and density are most reliable because they are quantitative and unique to each pure material. Scientists often use these constants to verify purity in laboratory tests.