What law that states the mass of the reactants is always equal to the mass of the products?
The law of conservation of mass.
This law was formally advanced during the late 18th century; Antoine Lavoisier and Mikhail Lomonosov were probably its principle authors. Given the absolute necessity that mass is conserved in a chemical reaction, one would think that it would have become a foundational principle of science much earlier in humanity's development.
Is mass always concerved in a physical reaction?