What is a characteristic feature of pure substances when heated?

Pure substances have a distinct characteristic where they melt and boil at specific temperatures. This is due to their consistent and uniform composition, meaning that every particle is identical. When heating a pure substance, it will change from solid to liquid at its melting point and from liquid to gas at its boiling point without any fluctuation in temperature during these phase changes. This property is fundamental in distinguishing pure substances from mixtures, which do not have fixed melting or boiling points due to the presence of multiple components that can disrupt the uniformity.

In contrast, the other options do not inherently describe the behavior of pure substances when heated. For instance, changing color can occur for various reasons and is not a definitive characteristic of pure substances. The production of gas may happen with certain substances but is not a general feature exclusive to purity. Dissolving in other substances is also variable and depends on the nature of the solvent and solute rather than the purity of the substance itself. Therefore, the ability to melt and boil at specific temperatures is a reliable and defining trait of pure substances.