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A solution can be defined as a homogeneous mixture of two or more substances. Generally, one thinks of a solution as liquid, but gaseous or solid forms are possible as well. Indeed, we can have solutions of gases in a gas, gases in a liquid, liquids in a liquid, solids in a liquid, and solids in a solid. A solution can have one or more solutes dissolved in a solvent. The solute is the substance that is dissolved; the solvent is the substance in which the solute is dissolved. In a solution, there is always less solute than solvent. There are two kinds of solid solutions: substitutional and interstitial. Figure 10.1 shows examples of each in a schematic manner. Figure 10.1(a) is of brass, which is a substitutional solid solution of zinc (the solute) in copper (the solvent). We call such an alloy substitutional because the solute atoms merely substitute for the solvent atoms in their normal positions. In a substitutional solution, the atomic sizes of the solute and solvent atoms are fairly close. The maximum size difference is approximately 15%. When the atomic sizes of the solute and solvent are very different, as in the case of carbon or nitrogen in iron, we get an interstitial solid solution. Figure 10.1(b) shows such a solid solution of carbon in iron. We call these solutions interstitial solid solutions because the solute atoms occupy interstitial positions in the solvent lattice.