3. Parting with sulphuric acid. Gold may be separated from silver, as well as from many other metals, by heating the alloy with concentrated sulphuric acid. This dissolves the silver, while the gold is not attacked.
Physical properties. Gold is a very heavy bright yellow metal, exceedingly malleable and ductile, and a good conductor of electricity. It is quite soft and is usually alloyed with copper or silver to give it the hardness required for most practical uses. The degree of fineness is expressed in terms of carats, pure gold being twenty-four carats; the gold used for jewelry is usually eighteen carats, eighteen parts being gold and six parts copper or silver. Gold coinage is 90% gold and 10% copper.
Chemical properties. Gold is not attacked by any one of the common acids; aqua regia easily dissolves it, forming gold chloride (AuCl3), which in turn combines with hydrochloric acid to form chlorauric acid (HAuCl4). Fused alkalis also attack it. Most oxidizing agents are without action upon it, and in general it is not an active element.
Compounds. The compounds of gold, though numerous and varied in character, are of comparatively little importance and need not be described in detail. The element forms two series of salts in which it acts as a metal: in the aurous series the gold is univalent, the chloride having the formula AuCl; in the auric series it is trivalent, auric chloride having the formula AuCl3. Gold also acts as an acid-forming element, forming such compounds as potassium aurate (KAuO2). Its compounds are very easily decomposed, however, metallic gold separating from them.
EXERCISES
1. From the method of preparation of platinum, what metal is likely to be alloyed with it?
2. The "platinum chloride" of the laboratory is made by dissolving platinum in aqua regia. What is the compound?
3. How would you expect potassium aurate and platinate to be formed? What precautions would this suggest in the use of platinum vessels?
4. Why must gold ores be roasted in the chlorination process?