What information does a UV-Vis spectrum provide?

A UV-Vis spectrum provides valuable information about how a substance interacts with ultraviolet and visible light. Specifically, it shows the absorbance of UV or visible light by the substance across different wavelengths. When a compound absorbs light, it can promote electrons to higher energy levels, which corresponds to specific wavelengths in the spectrum. This absorbance can help identify the presence of certain functional groups or confirm the identity of a compound based on the distinct peaks observed in its spectrum. Hence, the correct choice captures the fundamental purpose of UV-Vis spectroscopy in analytical chemistry, which is to analyze the electronic transitions of molecules.

In contrast, the other options discuss characteristics unrelated to what a UV-Vis spectrum measures; temperature, pH, and molecular weight are determined through different analytical methods and techniques.