Dissolution was an operation performed on a bench with a solvent, and it kept that practical sense throughout the history of the art even where it also carried a symbolic one.
What the word describes
Solutio means bringing a solid into a liquid state, either by dissolving it in a menstruum or by reducing it to a liquid with heat and moisture. The alchemical corpus is full of arguments about which solvent will act on which body, and those arguments are chemistry: a substance that dissolves in one liquid and not another is telling the operator something about itself.
The maxim that one should dissolve and coagulate, solve et coagula, pairs this operation with its opposite and is among the most quoted phrases in the literature. It describes an alternation, not a single step.
Where the number seven comes from
The scheme of exactly seven numbered operations, each assigned to a planet, is a modern arrangement. The historical texts do not agree on a count. Ripley's fifteenth-century Compound of Alchymy sets out twelve gates; other authors give other numbers, and the operations named in this atlas appear among them in different orders and different company.
The atlas records the seven-step sequence because it is the version in general circulation today, while recording that the underlying tradition was not this orderly.