Like if carbon dioxide is in the air how did we know it was CO2? We didn’t just zoom into the air and C bouncing around with 2 Os
In: 4
Carbon dioxide in the air:
People knew that fire can be suffocated, but it took a while to figure out why.
Someone figured out that there’s changes in the volume and density of the air that fire consumed. He found that something in the air gets heavier when that air is used for combustion. On the other hand, the burning medium (for example wood) gets lighter. The overall weight remains the same. Therefore, something in the air combines with something from the wood.
(other fuel gets heavier: iron wool for example catches oxygen and gets heavier while the air loses it).
After such research, eventually we knew that there’s oxygen and also carbon dioxide.
More experiments figured out how carbon dioxide behaves with other chemicals. It binds to calcium easily, and forms a solid compound when streamed through water containing calcium.
The amount of that calcium carbonate tells us how much CO2 was pumped through the calcium water, and can be used to measure the amount of CO2 in the air.
Today we use more sophisticated ways to measure the composition of air, using light as mentioned in the other comment. Shining special light through the air and measuring the resulting light can tell us what’s in the air, and even the percentage of single components.
The first step was figuring out that stuff breaks down into other stuff, and then figuring out that we can put it back together.
This meant that substances don’t just transform into other substances, but that there’s building blocks (the chemical elements).
People then tried to isolate new elements from natural substances. This cemented the theory.
Ever since then, scientists tried to find precise ways to analyse substances.
The classic way is to take a sample and to destruct it with mechanical procedures (like smashing or filtering), to burn them and to gather the ashes, also the gasses; the using acids and bases to “rip apart” chemical compounds, forming salts; then electrolysis to rip apart compounds in a different way. There’s also filter chromatography: like the rings coffee stains can make on a towel, solutions of substances travel differently through filter substrates. This also means the concentration of fractions of mixtures.
Eventually, scientists found connections between substances and the way they influence light. Substances and (elements themselves) absorb and reflect light differently. This means that the light that comes from a sample contains information about the sample. We figured out how to use this information.
Today there’s even more methods, mostly relying on the methods described above.
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