Iron and gold are different because iron rusts and gold does not.

铁和黄金不同，因为铁会生锈，而黄金不会。

So, when exposed to oxygen either in air or water the iron rusts and becomes iron oxide.

所以当暴露在氧气里时，无论是在空气中还是水中，铁会生锈并且变成氧化铁。

The damp air makes the iron rust; but when we cover it with black-lead, the air cannot reach it, and it keeps bright.

潮湿的空气会让铁生锈，但是只要用石墨盖住，铁就接触不到空气，会一直亮闪闪的。