What are the properties of pure substances?

Pure substances have defined and consistent physical properties, which is why they melt and boil at specific temperatures. This characteristic is a key identifier of purity; for instance, the melting point of pure ice is always 0°C under standard atmospheric pressure, and pure water boils at 100°C. In contrast, mixtures do not have specific melting or boiling points, as their behavior can vary depending on the composition of the mixture.

The other options do not accurately describe the properties of pure substances. For example, pure substances do not exist in multiple phases simultaneously, and they can be solids, liquids, or gases at room temperature, depending on the substance. Additionally, they do not lack specific melting or boiling points, as that would imply they are mixtures rather than pure substances.