You’ll be glad (or sad??) to hear that it wasn’t the ping pong balls that exploded. What we did in the demo at the end of lecture 3 is put some liquid nitrogen into a 2 litre plastic bottle and screw on the lid. if you watch the video again, you can see Rosy place the sealed bottle into the bin just before we tipped the ping pong balls on top of
Liquid nitrogen boils at -198.6C, so at room temperature it quickly becomes a gas. Nitrogen gas takes up almost 700 times more volume than the same mass of liquid nitrogen. This means as the liquid nitrogen boiled, it very quickly produced enough gas to increase the pressure in the bottle until it exploded. The resulting force blasted the ping pong balls into the air (and also destroyed the bucket we had inside the large bin!).
The estimated pressure in the bottle just before it exploded was about 150psi, or about 10 atmospheres, although different bottles explode at different pressures, so that’s why we added the ping pong balls and moved away from the bin as quick as we could!