I am reading a chemistry book printed in 1805. The chemical reaction equations are written using the equality symbol = instead of the arrow →, which is used in modern times. Anyway sometimes it is still possible to see the "old fashioned" way: http://www.jeron.je/anglia/learn/sec/science/changmat/page13.htm Does anyone know why the equality symbol was abandoned, and when did it happen in the history of chemistry? Are there reasons why this change was needed? I know only a little about chemistry, I think this is a very basic question, but I cannot seem to find the complete solution myself. I can think that maybe the = was replaced by → because chemical reaction equations are not mathematical equations, there is no equality in the equation in mathematical sense. If the chemical equations are not mathematics, then why the addition symbol + has not been replaced by something else? The addition is a mathematical operation, so should it be understood to mean also a chemical reaction? Something is added into something else, perhaps this is an universal concept applicable not just in mathematics.