Final answer:
The alchemists' most significant contribution was their acceptance of observation and experimentation, setting the stage for modern scientific inquiry and developments in chemistry such as Dalton's atomic theory.
Step-by-step explanation:
The most significant contribution to modern science made by alchemists was their widespread acceptance of observation and experimentation. Alchemists' pursuit to transform elements, such as changing lead into gold, underscored early experimentation, even though their secrecy meant many discoveries were not shared. However, as alchemy gave way to the scientific method and modern chemistry, the approach involving systematic observation and experimentation became fundamental. This methodology later paved the way for profound developments in chemistry, including John Dalton's atomic theory and Amedeo Avogadro's concept of the mole, leading to the formulation of Avogadro's number. Hence, while alchemy itself did not yield directly to scientific theories, it did establish the foundation of empirical research that allowed future chemists to build a more rigorous and quantitative science.