Alloys
An alloy is a metallic material with a crystal structure made by adding another metal or a nonmetal to a molten metal. Its properties depend on its composition and structure: an alloy is often harder and stronger than the pure metals, and some melt far below their components. Metals with similar atomic radii form a solid solution (a homogeneous alloy); when the radii differ sharply the result is a heterogeneous mixture, and when the electronegativities differ sharply a chemical compound (an intermetallic compound such as CuZn) forms. Bronze is Cu with Sn, brass is Cu with Zn, and steel and cast iron are Fe with C; duralumin is mostly Al with Cu, Mg and Mn and is used in aircraft building. In jewellery the fineness mark shows how many parts of pure gold are in each 1000 parts of an article: 585 means 58.5 % gold. To express alloy composition in percent, divide the mass of a component by the mass of the alloy and multiply by 100 %.
“Alloy detective”: the teacher provides a list of everyday objects (coin, key, spoon, a picture of a jewellery item). Groups guess which alloy each is made from (bronze, brass, steel, an aluminium alloy, nichrome) and explain why that alloy was chosen. No chemicals are used to test the objects.