Summary: Metallurgy
Elements around us are grouped as metals, nonmetals and metalloids. Metals are usually shiny, hard, malleable, ductile and good conductors, while nonmetals are dull, brittle and poor conductors. Every group has surprising exceptions: mercury is a liquid metal, sodium can be cut with a knife, iodine shines, and graphite conducts electricity.
Metals give away electrons easily, so they react with oxygen, water and acids. They do not all react equally. By letting one metal push another out of its salt solution, scientists built the reactivity series, from very active potassium at the top to calm gold at the bottom. Nonmetals behave the opposite way: they accept electrons.
When a metal hands electrons to a nonmetal, oppositely charged ions form and hold each other tightly. These ionic compounds, like common salt, are hard crystals with high melting points. They dissolve in water but not in kerosene, and they conduct electricity only when molten or dissolved, which is why they are called electrolytes.
Metallurgy is the science of getting pure metal out of its ore. First the unwanted gangue is removed by gravity, magnets, froth floatation or leaching. Then the metal is extracted: electrolysis for very reactive metals such as aluminium, roasting or calcination followed by reduction for middle metals like zinc, and simple heating for copper. Finally the metal is refined.
Metals slowly get eaten away by air and moisture — iron rusts, copper turns green and silver turns black. This corrosion wastes a lot of money. We protect metals by painting, galvanizing with zinc, tinning (kalhai), anodizing, electroplating and by turning them into alloys like stainless steel and bronze.
Key points to remember
- Mercury and gallium are liquid metals at room temperature; bromine is the only liquid nonmetal.
- Tungsten has the highest melting point among metals (3422 °C); sodium and potassium are soft enough to cut with a knife.
- Graphite is a nonmetal that conducts electricity; diamond is the hardest natural substance and conducts heat but not electricity.
- Sodium is stored in kerosene because it catches fire on contact with air.
- Al, Fe and Zn do not react with cold or hot water but react with steam to give metal oxide and hydrogen.
- Aqua regia is a 3:1 mixture of concentrated HCl and concentrated HNO3; it dissolves gold and platinum.
- A more reactive metal displaces a less reactive metal from its salt solution: Fe + CuSO4 → FeSO4 + Cu.
- Ionic compounds are hard, brittle, crystalline, have high melting points and conduct electricity only in molten or aqueous state.
- Minerals that can yield a metal economically are ores; the earthy impurities in them are called gangue.
- Ores are concentrated by gravity (Wilfley table, hydraulic), magnetic separation, froth floatation (sulphide ores) or leaching (bauxite).
- Highly reactive metals (Na, Ca, Mg, Al) are extracted by electrolytic reduction of their molten compounds.
- In aluminium extraction, cryolite and fluorspar lower the melting point of alumina from over 2000 °C to about 1000 °C.
- Sulphide ores are roasted (heated in excess air) and carbonate ores are calcined (heated in limited air) to get oxides.
- Galvanizing coats iron with zinc; tinning coats copper or brass vessels with tin; stainless steel and bronze are corrosion-resistant alloys.
