Microwave Oven and Volcanic Eruptions

This thread reminded me of a question that’s been plaguing me since I recently had the pleasure of scalding most of the skin off of my left hand.

My normal procedure for heating a bottle of formula for BabyPhotog is to fill a 2 cup glass Pyrex measuring cup with water and heat it in the microwave for 5 minutes. Upon removing the Pyrex from the microwave, the bottle is placed in the water until such time that the formula reaches the optimum temperature to not only satisfy BP, but also gently coax her into peaceful slumber.

Most of the time, all goes as expected. 5 minutes of ‘nuking’ produces boiling water into which the bottle is carefully placed.

Sometimes, however, after 5 minutes the water does not appear to be boiling - Hot and steamy? Yes. But boiling? No.

Once the bottle is placed in the water, though, the water will suddenly erupt into a violently rapid boil, splashing about and scalding those too slow to get their left hand out of the way.

So what gives? What is it about microwaves that causes this to happen, and why is it not consistent. Why is this energy sometimes released as normal boiling, but other times only released once the water is somehow disturbed?

You’ve superheated the water to above its boiling point. This can happen in any smooth container, in a microwave, or on a stove top. It’s just more commonly noticed with microwaves because people use more glass containers in them. You can prevent it from happening by putting a wooden spoon or stick in with the water. This provides the nucleation sites needed for normal boiling. Here’s some more on the subject: Superheating and microwave ovens

Thanks very much for the quick reply and the very informative link.