Alchemy covers several philosophical traditions spanning some four millennia and several continents, and it can be summed up in this way... Alchemy is the philosophy and practice of liberating a physical substance from its "fixed" physical properties.
It was believed that without achieving a form of enlightenment or inner spiritual revelation, alchemy would not be possible to the practitioner. There would be no hope of you transcending the fixed properties of the physical dimension so as to transform something like ordinary metal into gold without gaining access to the higher truths of this universe through spiritual practice and mastery. And so alchemists of the past used scientific processes, in other words techniques that could be replicated in a lab together with hermetic principals and spirituality. Those principals and spiritual truths are the basis upon which alchemy stands.