Have you ever seen someone breathe into a device like this? This person is breathing into a breathalyzer, a device used to detect the presence of ethanol (an alcohol) in a person’s breath. One version of the breathalyzer works by measuring the color change of a solution of the dichromate ion. In the presence of ethanol, the solution changes from orange to green as the dichromate ion is converted to a chromium(III) ion. The extent of the color change is then related to actual blood-alcohol content. The device is very accurate, because the chemical reaction on which the device is based is very predictable. This is an example of an oxidation-reduction reaction.
Courtesy of Senior Airman Natasha Stannard, US Air Force. commons.wikimedia.org/wiki/File:Breathalyzer_study.jpg. Public Domain.