'Lecture Demonstration'' is a short story by Hal Clement. It was first published in Astounding: The John W Campbell Memorial Anthology in 1973.
Dr LaVerne, a teacher with the College, takes a party of Mesklinite students on a geological expedition. Whilst examining a layer of rock, it collapses. Teacher and students fall into a cavern. They are unable to climb out; the limiting factor is time as the teacher is enclosed in a suit with a finite oxygen supply.
The students and teacher discuss various possibilities until the realise that they can raise the melting point of the surrounding ammonia 'snow' to the point where it soldifies. They climb out to safety.