Question

which metal is mixed with gold to make it hard

Answer 1

Pure gold is too soft for everyday wear, so it is alloyed with a mixture of metals like silver, copper, platinum, palladium, and zinc to give it strength and durability.

Answer 2

24 carat is pure gold with no other added metals. But sometimes other metals are added to make it hard, like in 18 carat gold which contains 75% other metals like copper or silver. Sometimes nickel and zinc are also used.