Glass sealed NO2

This is used to demonstrate how temperature affects chemical equilibrium. In the sealed tube, NO2 and N2O4 interconvert. Their ratio changes with temperature, producing different colors (more NO2 gives a brown color; more N2O4 gives a colorless/pale appearance). It’s a classic equilibrium demonstration.

You can see an example here:
https://www.youtube.com/watch?v=Vt9HyDnTZ6E