Sterilizing Water with Microwave

For years I've been lazy and used the microwave to sterilize water for primer sugar and to rehydrate water for yeast.  Never found any problems with yeast or beer doing it this way. I've always been curious if this changes anything in the water.

Not in a way that's different than any other heat source. Microwaves merely put energy into the water molecules, same as a flame.

Add a chop stick or something similar so you do not burn yourself.  Water in a glass container can get above boiling temp in a microwave and not boil, an accidental bump of the pyrex can start a vigorous boil and splash.  The chop stick gives a place for the boiling/bubbling to start so you can see that its hot.  This is really only true for distilled water but better safe than sorry

Amen to that ^^^ super heated water they call it. It will literally burn your face off! Mythbusters tested and proved it. I was afraid to microwave water for years lol

Ski12568 said:

Add a chop stick or something similar so you do not burn yourself.  Water in a glass container can get above boiling temp in a microwave and not boil, an accidental bump of the pyrex can start a vigorous boil and splash.  The chop stick gives a place for the boiling/bubbling to start so you can see that its hot.  This is really only true for distilled water but better safe than sorry

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It can happen with tap water which has been sitting for a while bit it won't happen with water straight from the tap which has too much air, or so I understand.

I know this from painful experience. I heated some tap water for coffee in a glass mug but forgot about it so I re-heated it after about half an hour. As I took it out of the microwave it exploded burning my wrist and leaving a scar. I won't do that again.

I do sterilize water in the microwave but I let it cool for a long time before I open the door. I also put a wooden coffee stirrer in. 

I had heard that even jiggling the cup when you remove it or breaking the surface tension, such as a spoon, adding sugar, etc. can cause the water to explode.  In example pour a soluble powder into the superheated water, will carry thousands of tiny air bubbles into the water. Each of these micro bubbles expands into a 1cm steam bubble, and the result is a huge "explosion" of hot froth.
Read more at http://www.snopes.com/science/microwave.asp

+1 on the wooden stirrer Roadrocket..