<P> Gold, a siderophile, is most commonly recovered by dissolving the ores in which it is found in a cyanide solution . The gold forms a dicyanoaurate (I), for example: 2 Au + H O + 1⁄2 O + KCN → 2 K (Au (CN)) + 2 KOH . Zinc is added to the mix and, being more reactive than gold, displaces the gold: 2 (Au (CN)) + Zn → K (Zn (CN)) + 2 Au . The gold precipitates out of solution as a sludge, and is filtered off and melted . </P> <P> Some general physical and chemical properties of light and heavy metals are summarised in the table . The comparison should be treated with caution since the terms light metal and heavy metal are not always consistently defined . Also the physical properties of hardness and tensile strength can vary widely depending on purity, grain size and pre-treatment . </P> <Table> Properties of light and heavy metals <Tr> <Th> Physical properties </Th> <Th> Light metals </Th> <Th> Heavy metals </Th> </Tr> <Tr> <Td> Density </Td> <Td> Usually lower </Td> <Td> Usually higher </Td> </Tr> <Tr> <Td> Hardness </Td> <Td> Tend to be soft, easily cut or bent </Td> <Td> Most are quite hard </Td> </Tr> <Tr> <Td> Thermal expansivity </Td> <Td> Mostly higher </Td> <Td> Mostly lower </Td> </Tr> <Tr> <Td> Melting point </Td> <Td> Mostly low </Td> <Td> Low to very high </Td> </Tr> <Tr> <Td> Tensile strength </Td> <Td> Mostly lower </Td> <Td> Mostly higher </Td> </Tr> <Tr> <Th> Chemical properties </Th> <Th> Light metals </Th> <Th> Heavy metals </Th> </Tr> <Tr> <Td> Periodic table location </Td> <Td> Most found in groups 1 and 2 </Td> <Td> Nearly all found in groups 3 through 16 </Td> </Tr> <Tr> <Td> Abundance in Earth's crust </Td> <Td> More abundant </Td> <Td> Less abundant </Td> </Tr> <Tr> <Td> Main occurrence (or source) </Td> <Td> Lithophiles </Td> <Td> Lithophiles or chalcophiles (Au is a siderophile) </Td> </Tr> <Tr> <Td> Reactivity </Td> <Td> More reactive </Td> <Td> Less reactive </Td> </Tr> <Tr> <Td> Sulfides </Td> <Td> Soluble to insoluble </Td> <Td> Extremely insoluble </Td> </Tr> <Tr> <Td> Hydroxides </Td> <Td> Soluble to insoluble </Td> <Td> Generally insoluble </Td> </Tr> <Tr> <Td> Salts </Td> <Td> Mostly form colourless solutions in water </Td> <Td> Mostly form coloured solutions in water </Td> </Tr> <Tr> <Td> Complexes </Td> <Td> Mostly colourless </Td> <Td> Mostly coloured </Td> </Tr> <Tr> <Td> Biological role </Td> <Td> Include macronutrients (Na, Mg, K, Ca) </Td> <Td> Include micronutrients (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo) </Td> </Tr> </Table> <Tr> <Th> Physical properties </Th> <Th> Light metals </Th> <Th> Heavy metals </Th> </Tr>

Where are the heaviest metals located in a periodic table