When you think of a solutions, a substance dissolved in water is usually the first thing that comes to mind. However, some solid solutions contain combinations of metals where one metal has been dissolved into another. Alloys like brass are common examples you encounter in everyday life. Solid solutions should not be confused with chemical compounds, which are in a separate and more specific category.
A solid solution is homogenous, meaning its composition is more or less the same throughout. Solid solutions are most likely to form when both the solute metal and the solvent metal have atoms of similar size, crystal structure and electronegativity. Electronegativity is a measurement of the extent to which an element "hogs" electrons when paired with other elements. Two kinds of solid solutions can form. In a substitutional solid, solute atoms substitute for solvent atoms in the crystal lattice. In an interstitial solid solution, by contrast, the atoms of the solvent metal are bigger than the solute and the solute atoms fit into the gaps or interstitial spaces between the solvent atoms.
A compound contains atoms of more than one element in a fixed proportion to each other. For example, water always has two times more hydrogen atoms than it does oxygen. Moreover, the atoms in a compound are joined together by bonds, i.e., interactions that hold the atoms of the compound together. The atoms have a definite relationship to each other in terms of how they are arranged in space.
Mixtures and Compounds
The components of a mixture can be separated by physical means, while the components of a compound can only be separated by chemical reactions that break and/or form bonds. If you mix iron filings and dirt, for example, you have a mixture that you can separate with a magnet. The chemical composition of the iron filings and the dirt has not changed. If you want to break water into hydrogen and oxygen, however, you would need to break the chemical bonds that hold the water molecules together. A solid solution is a type of mixture that can be separated by physical means, so it does not fall into the same category as a compound.
The atoms in a compound are always present in a fixed ratio, but the atoms in a solid solution can be present in a variable ratio. Not all alloys of brass, for example, have the same zinc and copper content. Moreover, while the solid solution has the same crystal structure as the pure solid, the spatial arrangement of the atoms is not unchanging as it is in a compound. A water molecule always has the same spatial arrangement for its component atoms. However, the atoms in a solid solution can be substituted for each other in various places.