Substances made of different molecules absorb radiation at different energies; this can be used by chemists to discover more about the characteristics of molecules that make up various substances. Through infrared spectroscopy, we can learn what regions of radiation a substance absorbs. Open up the IR Windows learning tool to explore the IR spectra of various atmospheric gases.

Question: Which gas molecules absorb IR radiation of higher energy: Water or CFC-12?

In the top left corner of the learning tool, click on Water and CFC-12 to add their infrared spectra to the graph below.

Which region of the spectrum is higher in energy? Notice that the scale is in wavenumbers. Recall that the shorter the wavelength of electromagnetic radiation, the greater its frequency and energy.

CFCs absorb IR that has a wavenumber of about 1000 cm-1. However, water absorbs IR of much higher wavenumbers (and higher energy), above 3500 cm-1.