24K gold is pure gold. The karat system indicates the number of parts out of 24 which are gold within an alloy. Clearly, there is no such thing as 24K white gold. I’ve summarized briefly some information about gold alloys at:
High karat gold has a deeper and richer color, hence is used in many designs to enhance the design and add more contrast. Because of its softness, it is used sometimes to wrap stones. See some examples at:
Sarit.

24 karat gold is not 100% pure–it is 99 to 99.9% pure depending on the laws of the country of origin. You won’t find white gold above 18 karat for just the reason you list.

24 karat gold is generally not the best choice for jewelry because of its softness, but you will find some examples. Most fine piece of jewelry are 18k.