Super-heating

Normal boiling takes place when bubbles of water vapor form throughout the liquid. The process is facilitated by any small sharp objects that act as condensation nuclei - lowering the threshold energy for the formation of tiny bubbles.

Heating clean water in a very clean test tube with a Bunsen flame at the bottom may cause bumping. Hot water is ejected from the tube by large rising bubbles. The water at the bottom has become super heated (over 100°C) because of a lack of condensation nuclei. Boiling eventually takes place suddenly, with violent consequences.

Super heating is a common problem with clean containers of water in a microwave oven. If water becomes superheated any small disturbance, tap, addition of instant coffee, etc. can cause catastrophic boiling - throwing superheated water over the unsuspecting.

Do take care - this is not a joke!

Condensation nuclei are important in many situations.

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