24 carat gold is made up of pure gold so it is the pure mettalic molten gold which can be considered as an element and hence is quite soft but in 22 carat gold the gold is onl 22 carat and the rest 2 carats are the other metals like iron,silver ,copper etc. which is mixed to make the gold more harder and convertible into ornamental forms .
So 22 carat gold is a type of mixture