Alchemy is a mystical superscience involving the combination of chemistry with supernatural powers. An alchemist can create reactions between chemicals that will invoke the inherent powers of those chemicals, creating a wide variety of effects, mostly by creating matter that has powers inherited from the reactions, either by blending of or subtracting from the component parts.
See the materials page for a good start on materials and their aspects.
Basics of alchemical reactions:
Purifying a reactant will create powerful single-sphere effects.
Blending is akin to combining spheres with magic.
Every state of matter, classical or not, has its own supernatural properties.