No definite composition. Definitions depend upon scale. A homogeneous mixture may be heterogeneous if you look closely enough. The solute vs the solvent. The history of chemistry is as long as human history.
Progress is measured by advances in chemistry.
Recent progress in science including chemistry is a consequence of the use of the scientific method . If truly repeatable, it becomes a law, which is a concise verbal or mathematical statement or a relation that is observable in nature or in experiments. A theory is a unifying principle that explains a large number of observations. A model provides a mental framework to serve as a foundation for a detailed explanation. All progress does not follow the scientific method, for often great advances have been made by chance .