Silver and gold: properties and uses
Silver (Ag, Z = 47) and gold (Au, Z = 79) are in the side subgroup of group I together with copper; both stand after hydrogen in the activity series, so they do not displace hydrogen from acids. Silver is a white, soft metal and the best conductor of electricity and heat; in compounds it is usually +1 (Ag⁺). It does not oxidise in air, but turns black under sulfur compounds (Ag₂S). Silver dissolves in concentrated HNO₃: Ag + 2HNO₃ → AgNO₃ + NO₂↑ + H₂O. AgNO₃ is a reagent for halide ions: Ag⁺ + Cl⁻ → AgCl↓ (white), AgBr is pale yellow; because AgBr is light-sensitive it has been used in photographic materials. Ag⁺ ions kill microbes. Gold is a yellow, very malleable metal of high density (19.3 g/cm³), occurs in nature mostly free (native), does not oxidise, and of the acids it dissolves only in aqua regia (a 1 : 3 mixture of HNO₃ and HCl). In industry gold is separated from ore by cyanidation; cyanides are extremely poisonous, so the process is strictly controlled. Because gold is soft, jewellery uses alloys with copper or silver; for example in an item stamped 750 gold makes up 75 %.
Make a table in class for Cu, Ag, Au: colour, density, electrical conductivity, behaviour towards acids, uses. Write why gold coins and jewellery still shine after thousands of years. Chemical experiments (aqua regia, silver salts) are done only by the teacher.