This document provides an overview of alloys and solid solutions. It defines an alloy as a substance composed of two or more metals. There are three possible phases in the solid state: pure metal, compound/intermediate alloy phase, and solid solution. Solid solutions occur when an alloying element dissolves into the crystal structure of another metal. There are two types of solid solutions - substitutional, where solvent atoms are replaced by solute atoms, and interstitial, where small atoms fit into the spaces of the solvent's lattice. Factors like crystal structure, size, chemical affinity, and valence influence solubility ranges in substitutional solutions based on Hume-Rothery rules.