I guess you are referring to one of our Physics Van demonstrations which took place at Engineering Open House - where a large can of liquid nitrogen was poured into a bucket of soapy water, resulting in a huge explosion of bubbles!
So what actually happened?
The water in the bucket is actually much, much warmer than the liquid nitrogen. This is because the liquid nitrogen is kept at a really low temperature for it to remain liquid. When liquid nitrogen touches the water, it suddenly warms up and becomes a gas! Gases take up much more space than liquids. Thus, all of a sudden, the nitrogen takes up at lot of space. The soap lets us see the nitrogen gas because it makes bubbles - lots of them, in fact - and it all happens so quickly that we see an explosion of bubbles!
I hope this helps, and thanks for your question!
(published on 03/29/11)