Copyright © 2007 Dorling Kindersley
An alloy is a mixture of metals, or of metals and other substances. Mixing metals and other elements in alloys can improve their properties. The alloy bronze is a mixture of the metals copper and tin. It is resistant to water corrosion and is used in outdoor structures.
Table 25. ALLOYS
A pure metal has identical atoms arranged in regular layers. The layers slide over each other easily. Alloys are harder and stronger because the different-sized atoms of the mixed metals make the atomic layers less regular, so they cannot slide as easily.
The different sizes of atoms in an alloy make their arrangement less regular than a pure metal. This makes the bonds between the atoms weaker, and lowers the melting point. Alloys that melt easily, such as SOLDER, have important uses.
About 6,000 years ago, early peoples made the alloy bronze by roasting together copper and tin ores (minerals). Bronze is stronger and longer-lasting than pure copper. This period in history when bronze was the main material used is called the Bronze Age.
Lead is a heavy, soft metal, and melts at a low temperature, 622°F (328°C). By adding tin to make the alloy solder, the melting point is lowered further.
Flux is any substance that stops a metal from oxidizing (combining with oxygen), such as salt. Most metals oxidize in air—the process is speeded up with heat. When a plumber solders together lengths of copper pipe, he or she coats the surface with flux to stop the copper from oxidizing. Otherwise, the solder would not stick, and the pipes could not be joined.