How did Joseph Black’s research show that gases can be chemical substances?

Joseph Black’s research fundamentally changed the understanding of gases by demonstrating that they could be distinct chemical substances rather than merely atmospheric air in different states of purity.1

Through a series of precise, quantitative experiments using a balance, Black studied the chemical properties of substances like magnesia alba (magnesium carbonate) and chalk (calcium carbonate). He observed that when these compounds were heated or treated with acids, they released a specific gas, which he termed "fixed air" (now known as carbon dioxide).123

Key aspects of his discovery included:
* Quantitative Evidence: By weighing his samples before and after heating, Black proved that the weight lost during the process was due to the gas being driven off.1
* Chemical Recombination: He demonstrated that the original substance could be recreated by reintroducing the "fixed air" to the remaining product, proving the gas was a constituent part of the compound.1
* Distinction from Atmosphere: He showed that this gas was not simply common atmospheric air, establishing that gases possess unique chemical identities.14

This work laid the foundation for pneumatic chemistry and inspired further characterization of other gases.14