Question

Why did scientists pre-Lavoisier believe that matter was destroyed in the burning fire experiment?

Answer 1

Before Lavoisier, the father of Chemistry, scientists thought that every combustible substance lost matter when burned because they observed that a residue of less weight was left after combustion.

They reasoned that this loss of matter was due to the loss of a substance they called phlogiston to the air when the substance burned to make fire. 

Lavoisier proposed a theory in 1777 that excluded phlogiston and explained that combustion was the reaction between a metal or an organic substance with a part of air he later called oxygéne, from the Greek words for "acid generator". He could propose that matter was neither created nor destroyed in chemical reactions but transformed.