Final answer:
UV-vis spectrophotometry can be used to determine if two substances are unique by analyzing their unique absorption spectra at specific wavelengths, which are influenced by the presence of chromophores in the molecules. However, if two substances have similar chromophores, their spectra might be similar, making it difficult to distinguish between them using this technique alone.
Step-by-step explanation:
UV-vis spectrophotometry can be used to determine whether two substances are unique by analyzing their absorption spectra. When a substance absorbs light in the UV or visible spectrum, it does so at specific wavelengths that are characteristic of its molecular structure, particularly if the molecules contain chromophores. Chromophores are parts of molecules that absorb light; different chromophores absorb at different wavelengths. The unique pattern of absorption, or the lack thereof, at certain wavelengths can distinguish one substance from another.
For example, a molecule with a maximum absorbance at 630 nm, in the orange range of the spectrum, will appear blue. This is because the compound is absorbing light in the orange range and reflecting or transmitting light in the blue range. Such characteristic absorption helps in identifying the substance. However, it's worth noting that if two different substances have the same chromophores and similar structures, they might exhibit very similar UV-vis spectra, making it difficult to distinguish between them using only UV-vis spectrophotometry.
Nevertheless, UV-vis spectroscopy is a widely useful technique, particularly in the study of substances with conjugated systems, such as many organic and biological molecules. It is based on the principle of the Beer-Lambert Law, which relates absorbance to concentration. This method, therefore, not only helps in identifying substances but also in quantifying them.