Re: Brass (alloy) physical vs chemical combination

Tiffany,

From the point of view of a chemist, matter comes in three forms: elements, compounds and mixtures. Elements are just that, pure elements in whatever form they naturally come in. Compounds make up a great deal of what we see around us. They are combinations of elements that are chemically bound together, and so must be chemically separated. Mixtures are combinations of elements that can be separated by physical means. One example in the chemistry text I am looking at right now is of iron filings in glycerol (an ingredient in hand lotion). The iron filings can be separated by application of a magnetic field.

In the case of any alloy, we are looking at a mixture of metals (mostly). These can can be in several different forms. A solid solution is mostly one kind of metal with occasional substitutions of the guest metal in the crystal lattice. An intermetallic compound is a distict material from any of the metals that comprise it, often having a completely new crystal structure. Other alloys are heterogeneous mixtures, where small clusters of one metal exist within a matrix of the other metal.

What makes alloys different than a chemical compound? The metals in alloys can be separated by purely physical processes, like heating and cooling the alloy (including melting). A chemical compound cannot be separated so easily. For example, water is made of two hydrogen atoms and one oxygen atom. If you freeze water, it becomes ice, but that is still comprised of water molecules, each with two hydrogen atoms and one oxygen atom. If you vaporize it, the water becomes steam, but it is still molecules.

Alloys can also have varied compositions. The ratio of one metal to another is not necessarily any particular number. For example, an alloy of nickel and aluminum can have any ratio of nickel to aluminium atoms, and the various materials have radically different properties (Ni₃Al is magnetic, but NiAl is not). Hydrogen and oxygen only combine in certain ratios: 2 to 1 for water (H₂O), 1 to 1 for hydrogen peroxide (H₂O₂).

I hope this helps. Most of this information is available in an elementary chemistry text, if you know where to look. Alloys in general are one focus of a field called Materials Science, which attempts to study, explain and predict the physical properties of materials, typically with an aim towards application.