It seems like you might be looking for a term related to the mixing of substances, possibly in a scientific context. A suitable word to fill in the blank could be "dissolution" if you're referring to a solid dissolving in a liquid. Alternatively, "solution" could also fit, as it refers to a homogeneous mixture of substances.
If you're looking for a more general term, "blending" or "compound" might also work.
Could you clarify the context for a more accurate suggestion?