Final answer:
Lavoisier assumed that oxygen was necessary for the products of the acid, as his research indicated its critical role in both combustion and respiration, leading to the production of carbon dioxide.
Step-by-step explanation:
Lavoisier assumed that oxygen was necessary for the products of the acid. His experiments on the combustion of substances and the processes of respiration in animals led him to conclude that oxygen played a critical role in these chemical reactions.
For instance, when studying respiration, Lavoisier compared the process to the burning of charcoal, which consumes oxygen and produces carbon dioxide. Similarly, the general combustion of hydrocarbons also results in the production of carbon dioxide and water, both of which involve oxygen. Regarding acids, Lavoisier's oxygen theory postulated that all acids contained oxygen, which was an essential element to their chemistry.