You are no doubt familiar with the chemical compound known as carbon dioxide. From the time you were a child, you learned that we breath in oxygen and breath out carbon dioxide (a gross simplification of what actually happens, but nevertheless effective for teaching the respiratory system to a younger and probably cuter version of you). You might even know that the chemical formula for carbon dioxide is CO2. If not, you could probably deduce it from the name.
But has this compound always been known by this name?
It has not. Discovered in the middle 1700s, carbon dioxide was first known as “fixed air”. It later became known as carbonic acid gas because when it was bubbled through water it made an acidic solution. It didn’t get the name carbon dioxide until the combined observations of Joseph Proust, Joseph Priestley, and Antoine Lavoisier in 1794 led to what we now call the Law of Definite Proportions.
The Law of Definite Proportions says that a given compound will have the same proportion of its constituent elements regardless of the size or location of the sample. In other words, “fixed air” will consist of one carbon and two oxygen molecules whether it is the product of your breathing, or it comes from a campfire, or it is produced when a 4th grader mixes vinegar and baking soda in her volcano science fair project.
