Why does tarnish disappear from a sterling silver ring when you wear it?

I have on more than one occasion stuck my hand, on which I wear a sterling silver band, in cleaner or bleach and the ring turns black. I know it's a chemical reaction of some sort, but I've learned not to panic because it always goes away after awhile. Usually about a week or so. I wear the ring daily. Why does it go away? Just curious.