Leaving Bleach in a Measuring Cup

Advice from Dr. Laundry
Q.

I left a measuring cup with liquid bleach in it for a few days. When I came back, it had crystallized.  I was wondering – can I still use it?  Should I just add water?

A.

This is a really unusual question!  The crystals you found in your measuring cup are salt crystals; they are what remain after the bleach breaks down into salt + water, and the water evaporates.  The breakdown of bleach into salt and water is the primary reaction that makes it environmentally friendly.  However, it’s not reversible – adding water to the crystals will just make salt water again, not bleach.  For your measuring cup, simply rinse away the crystals before using more bleach. 

Has anyone else had this happen to them?