What distinguishes a titrant from an analyte?

The distinction highlighted in the answer pertains to the roles that the titrant and analyte play in a titration process. The titrant is specifically defined as a solution that has a known concentration of a particular reagent; it is added to react with an analyte during the titration. The analyte, on the other hand, is the substance whose concentration or amount is being determined through this process.

In a titration, the known concentration of the titrant allows for the calculation of the unknown concentration of the analyte based on the volume of titrant required to reach a specific reaction endpoint, typically indicated by a color change due to a suitable indicator.

The other options introduce misconceptions about these definitions. The titrant is not the same as the analyte, nor is it only found in excess or exclusively a gas. These characteristics can differ depending on the specific reaction and method being employed. Hence, understanding the roles of these substances in the titration context is essential in analytical chemistry.