18 carat and 22 carat gold alloys are almost completely resistant to chemical attack and 9ct alloys are much less resistant. Nine carat alloys can dull or even blacken from exposure to chemicals in the atmosphere, and might discolour in contact with household chemicals. 9ct chain is usually finished with a deeper yellow 18ct or 22ct coating by manufacturers, which is why the bright finish on a new 9ct chain gradually tones down to match other jewellery that you wear on a daily basis.
The original bright finish can usually be restored quite inexpensively by a working jeweller for little cost should your chain become chemically or mechanically damaged over the years.

Hardness
As well as affecting physical properties, alloying gold generally increases the strength and hardness, with some reduction in malleability / ductility. The silver atom is slightly larger than that of gold, so alloying gold with silver gives a moderate improvement in strength and hardness. The copper atom is significantly smaller than that of gold and so it has a greater effect on strengthening gold than silver, as it distorts the gold crystal lattice more. Thus reducing caratage from 24 carats through 22 ct down to 18 carat gold results in stronger and harder alloys, but alloying beyond 18 ct down to 9 carat does not have much further effect.